How stupid ?

So I get a load of virus laden email sent to me during this last outbreak. Now I’m not stupid, and I ensure that I delete the lot. I regularly run full scans of both my drives, and they are always clean.
So.. doing some clearing out of files, I delete the ‘trash’ of all my mailboxes – which would be where the deleted infected mails would have been. I then run the Windows Disk Cleanup and defrag the drive. A day or two later, I ran a utility (two in fact) that promise to delete all the stuff that windows keeps even though it tells you it has been cleaned up. With me ?
I’ve deleted, deleted, cleaned, and erased a whole stack of files, among which the viruses were.
Following a thread on a newsgroup, I decide to perform an online scan (even though my AVG is bang uptodate) and it reports that I have viruses. WTF ???? These are sitting in a /TEMP folder. Now I did not put them there, Windows did. It didn’t tell me, it did sod all about them.
So now I’ve not just deleted them, I’ve shredded them also.

I wasn’t infected, and am unlikely to be by an email virus (yes, I know they can arrive in different ways), but wtf is Windows doing ? Stupid bloody OS.