I meant to clarify that as "hardly any data worth keeping" - i.e. personal documents, it still feels the need to scan my 80GB collection of legitimately acquired MP3s, the MSDN content installations and of course, the tens of gigabytes that Microsoft Windows likes to amass in the WinSxS directory.
Well obviously it's going to scan all your MP3s, one of those might contain viral code. As for WinSxS, it's a bunch of hard links to files elsewhere, it doesn't really occupy "tens of gigabytes"
If there's nothing worth keeping though I don't really see the point of trying to disinfect the machine, reformatting and reinstalling is really the only sane option anyway. And this time leave UAC on and actually use a proper on-access virus scanner at all times.