XP isn't known for such on good hardware. You should do the usual tests by removing the case cover to see if there is a heat issue. If the lockups subside, then you talk to the machine's supplier.

Also, since SYM(antec) is a possible issue, remove it and try Free AVG for a while from http://www.grisoft.com Let me be clear that I have no cure for Symantec's 2004 software.