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Hi

What Windows Service Pack is your XP at? if not at SP3 then do update as many applications are more geared to SP3 files.

Is your Symantec up to date?
Have you installed any new software just before this happened? if so what and maybe uninstall it to see if its conflicting (especially any security apps)

What I would first try is to boot into Safe Mode (F8 at boot and choose Safe Mode with networking) and see if your PC doesn't have any issues, if ok then boot back into normal mode and then click Start > Run > type without quotes "msconfig" and hit enter, then click the Startup tab and disable all bar any Norton/Symantec items and reboot.

Is your PC and Norton/Symantec running ok?

If it is then you likely have another application causing the issue, to find out what, enter msconfig again and enable only one item at a time and reboot and check your Antivirus is working ok, and then if ok enable another and so on until you find a cause.

If you dont find a cause I would suggest running the Malware Read me and Run Me guide in the malware forum and post and attach the logs in that forum.