It could well be that the extra "space" is comprised of uncompressed files that are being scanned, so whilst they are compressed they are under 80gig, but when uncompressed and scanned they become much bigger. Do you have a lot of compressed files?
I am using Avast anti-virus (fully updated) and have been for some time now. I've noticed that within the past year or so, it's been detecting more hardrive space than I actually have. I now have 2 80 gig Western Digital drives, but whenever I do a virus scan, Avast is telling me that it has scanned 200+ gigs of files and drive space. Even before I installed the second 80 gig drive, it was 'scanning' about 120 gigs of space.
I'm on a WAP TKIP wireless network with 1 other PC and a Mac, but I've never really (and by "never really" I mean never, nada, have not) set up for our computers to share files or to even be networked in any way. Its simply set up so that we all have internet access through a router to the cable/broadband.
I'm firewalled via Zone Alarm and have performed numerous online security checks to see if I have any leaks in my security and everything seems to be in proper and working order.
Could someone have hacked my computer and I don't know about it. I've checked task manager and even installed Iarsn Task Info to see if any weird programs are running in the background, but everything seems normal. I've run internet checks on all unfamiliar processes and everything seems to be legit. If anyone has any insight on this, it would be greatly appreciated, if only to put my mind at ease.