Spoke too soon - XP still fecked

I really shouldn't have said the PC was fixed .......

No problems for 2 days then this morning all my old problems are back.

"My Computer" won't open, Internet Explorer won't start (although Firefox does), Eudora hangs and I've got the blue error screen again.

I know nothing was loaded or changed since I installed the USB network adapter which seemed to solve my problems. The PC has been rebooted a number of times between "resolution" and today with no problems.

So WTF can it be? Tried a restore to yesterday with no effect. The latest Norton Ghost recovery point was even earlier so that suggests that nothing has happened to change any significant controls or settings.............

Any further ideas apart from those already suggested?