it sounds to me like a power management issue... if i remember correctly there was actually a ptch for this problem... cause microsoft found out that if you leave the power on at the login screen ad the computer sleeps, it will lock up. check microsoft.com for an update, cause i think there is one... if not, turn off the sleep mode (you never want it to shut off your hdd, cause at the welcome screen if it shuts them off your done)

i love the new blue screen of death that comes with xp... microsoft claimed to have removed it, but ther is a new one that kills your computer and performs a physical memory dump when the system becomes unstable (usually from driver conflicts) now THAT is a screen i like to see.