freeze up

hi i run win xp pro an i recently formated an reloaded xp pro done all my updates etc its been probabbbly 1 month any way for some reason i keep locking up even when in very small programs such as paint the strange thing is it never does it from my page only does it from wifes an kids well wife an boys i have mmine wifes boys girls work an guest an yes it is totaly froze number lock zero responce ctrl alt del no responce have to reboot mine an wife is administrator rest are limited have not installed any new software an i reformatted becuase ther e was a virus that kept popping up on wifes side before an mine an my antivirus said it was quarenteened so i totly formatted hardrive an reloaded an my antivirus says i am clean an i dont show any hardware conflicts any suggestions