Sounds like you don't need access to any of your currently installed files anyway...the easy solution to your problem is simply to reinstall the operating system. Attempts to actually remove the virus would be a substantial undertaking (viruses are much more advanced than they were just five years ago or so) that requires a pretty in depth knowledge of windows registries, your web browser's interaction with the operating system, and windows start up procedures. If you do choose this option, it will probably have to be performed in safe mode as (well written) modern viruses integrate themselves into core windows systems and are almost impossible to remove in a normal start-up. A CD should have come with your computer to do just that (or at least 'Restore' the operating system to factory defaults). If you don't have that, you can always download a copy via bittorrent...perhaps not technically 'legal', but who cares. Or better yet, you can do what I've done and download Linux (Ubuntu installs easy), it's free and perfectly legal to download (not that being legal to download makes it better...but so many other things do, that's just icing on the cake). If you can't find a computer to download Ubuntu from and burn it to CD, they will send you one for free (but it takes 4-10 weeks), by copyright law they only have to offer the operating system as a free download and are free to sell it and charge shipping when mailed to you on a CD (provided they offer free downloads), but despite my occasional complaints about what Ubuntu does under the hood, Canonical is a good company, they do their best to make the operating system freely and easily available to anyone who wants it. If you need to request a free CD, you can start here, they'll ask for a description of the reason you can't download it, but if you have a legit reason, they'll send you a CD:
Alternatively, you could buy a Ubuntu/Kubuntu (Kubuntu is the same operating system, just with a different graphical user interface) bundle for a 4 quid (a little over six dollar) donation and have it shipped much faster. Last I checked, the money goes to helping send out free CD's to other people who can't download the operating system.
If you reinstall your operating system using any option other than the CD shipped with the computer and have a laptop, you may sometimes (but not always, depends on the computer) have to find some drivers after the install, regardless of whether your choose Windows or Linux; most functions will work just fine but there may be issues with things like sound, webcams, and ports.
This is all good stuff Trevor. I'm not sure if GiC appreciates the leap in a unix for most folks though . . .
I haven't installed Linux on a box in probably over a decade, is Ubuntu user-friendly that your "proverbial grandmother" could use it easily?
When Apple went to a BSD-based OS, I quit using Linux on my personal computers.
I could walk my 'proverbial grandmother' through an installation of the operating system over the phone; though as I said before, a few things might not work right if you have a laptop, especially a newer one...if you have a desktop I doubt there'll be any problems. Personal problems I had with my laptop, which I bought at Best Buy, took straight home, and immediately formatted all hard drive partitions and installed Linux, were that while the sound worked, the head phone port wouldn't mute the sound and the webcam didn't work. There are fixes for both, but they were somewhat involved; in fact, Ubuntu 10.10 fixed the sound by itself (I had problems under 10.04), webcam will probably be fixed under the next update, Canonical places a high priority on ease of install and use, including making sure peripherals work.
Before coming back to Linux as my primary operating system about a year ago I had not used it on a regular basis (only for programming) since about 2003 when in college, Ubuntu 10.10 is not Slackware 8.0...which is what I had used back then. It virtually installs on it's own, you won't have problems like your mouse or usb ports not working, and you don't have to spend hours editing configuration files just to get basic operability; I remember the first time I installed Slackware on a laptop, the mouse pad didn't work, I spent hours playing with configuration files and downloading drivers (some of which were still incomplete) on the command line via Lynx just to get it working, this was after spending hours getting the GUI to work with the graphics card. I was in college at the time, so I really didn't mind, it was a good learning experience and at the end of the day I had a great operating system, vastly superior to Windows ME which was what MS's latest contribution at the time.
But Ubuntu's not like that, it's come a long ways in user experience; however, as a word of warning, if you use a different flavor of Linux, you might not like Ubuntu once you get it up and running, over the past year or two they've moved many of the configuration folders from their traditional locations, the default install doesn't even display configuration files but compiled binaries, you often have to create the config files from scratch to use them in some instances, in other instances you have to reconfigure the software or even download an update to be able to use a config file. The change from GRUB to GRUB 2 was especially bad.
But, to the average user, most of this will never make a difference and they'll appreciate being able make changes from GUI toolboxes under the preference menu; they may be more limited, but they are easy to use. As Entscheidungsproblem said, the biggest challenge is just getting used to the new software, but it's often better. There's no internet explorer, but Chrome runs nicely (firefox less so), unless you're willing to fork out the money for Adobe's Photoshop, it's hard to compete with GIMP, VLC is head and shoulders above Windows Media Player, and if you do anything more serious with your computer, comparing the programing and software development tools available in Linux with those in Windows is like comparing IBM's latest supercomputer to Microsoft's Kin phone. Maple and Matlab are both available for Linux if you use those programs, but the free Maxima will serve most people's needs.
The only windows software I miss is the Office Suite and that's only because I work with so many Excel spreadsheets that have macros in them, I can still run windows in a virtual box when I need to work on those spreadsheets, otherwise I just use Google Documents...haven't really seen the need for OpenOffice.
Deal breakers would be if you play a lot of games, almost all of which are released only for Windows (with a few making to to OSX), or if you use AutoCad on a very regular basis...but if you just do these things occasionally, there's no reason not to use VMWare and do it in a virtual box.