It all started with a Windows Protection Error

Win 98 SE on normal restart displays BBS (big blue screen) with Windows Protection Error. Very difficult to bring back up (like 50 tries) and then prone to instability (freezing mostly). It seemed to be related to vid so I replaced the AGP card(w/GEForce 3ti). Then the HD developed bad spots so I replaced that. Continued instability resulted in a complete teardown, deleteed and reset partitions. Installed Win XP pro sp2 and one device at a time with correct drivers. It ran well for about 8 hours but it then began to hang again.
This problem makes the box all but unusable. Any suggestions?

P4 1.6 256 mb DDR