I would suggest getting an Ubuntu live-boot CD, booting from that, then inserting and formatting a USB (with gparted, it's fairly straightforward), before copying over whatever files you want to keep. For scanning, you can use either Avast! or ClamAV, they may even happen to be in the repositories.
All files with the extensions .jpg, .jpeg, .png, .gif, .mp3, .ogg, .mp4, .wma, .avi and .mkv are safe to copy over without scanning. I am not quite sure about .pdf - probably safe. .doc, .xls and .ppt, as well as any other Office files are possible to infect with various rather nasty scripts - either scan those, or open them in OpenOffice and save as .html (and hope that the translation borks the malware). .exes can generally be assumed to be infected, so I'd advise not taking those at all, even if scans show them as clean.
EDIT: Oh, and if your computer can run it, go for Win7.
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