Thanks hugely Essexboy! Combofix found a root kit. This explains the clean scans and then reappearances of this nasty little ****. I think I'm good now although I haven't been running the machine for long now. I can now boot into safemode which I wasn't able to before. System is snappier and no more warnings. Would combo fix have remedied the situation in your opinion?
"c:\documents and settings\Sean\Application Data\inst.exe
"Infected copy of c:\windows\system32\DRIVERS\atapi.sys was found and disinfected
Restored copy from - Kitty ate it :p "
My only concern presently is that XP firewall shuts off momentarily after logging in but then again it's been doing that for well over a year now. Just an MS glitch? I think so. I think I'll check my router settings while I'm at it.
Thanks again.