I would say it is not possible for a machine, not used in any length of time, not connected to any internet source, to get any sort of malware.
It's certainly strange, and especially so since I didn't realise AVG checked the boot process, unless you mean the system by-passed POST and then the warning displayed as XP was loading.
Can you remember the exact wording of the warning?
last night i fired up my old Windows machine(running XP) which had not been turned on in almost a month. when if started to boot up, i got a window from AVG saying unauthorized access to the computer had been detected--send report? it then crashed and tried to reboot SIX times! i finally got it going, ran an AV scan which found nothing....my question is, is it possible to get malware installed on your computer without it being on? i know it sounds ludicrous, but have not had problems with it before , other than it being an 8-year old Compaq with 512 ram and 80 gig HD...i recently got an imac so the reason i keep the old one is around is to try beta software and such...ironically, the reason i fired it up was to download saluto to it....