XP Home Login / Immediate Log off

Client has XP Home. Was a clean install. All worked well. Had a situation where the users would log in at the welcome screen and then XP would immediately log them off, automatically. This did the same thing in safe mode. No new programs installed, no new hardware. Attempted recovery console numerous times (checkdisk, fixboot, etc) to no avail. After extensive online research and talking with those with more knowledge than I, I decided to reformat and reinstall. All is fine now. My question is what could have caused this? I've been to two online bulletin boards, MSFT tech net, books, and all and can not find an answer. Any ideas? Running an ECS K7S5A Mobo / AMD XP 1800 chip / 512 DDR. No other problems to date. The only program that had problems was "grokster", the music download, but I do not think this would cause the situation to the best of my knowledge, but figured I'd mention it anyway. Thanks in advance for your time and efforts!!