WinXP shutdown problem

My HP Pavilion 7946 has preinstalled WinXP, which I have updated with SP2. I have run Microsoft Windows Update regularly and installed all recommended & relevant updates.

I run latest Norton antivirus + Internet Security, with auto-update enabled. Regular Spybot + Lavasoft checks show no problems.

I also clear out temp internet files and the temp directory regularly.

I have also downloaded all recommended software updates from Hewlett Packard.

Machine is networked to 1 other, an older machine running Windows 98 1st edition. They have been networked for 2 years, with the Hewlett Packard acting as the master and internet gateway. The HP uses NTFS file system, but the slave machine uses FAT (being win98).

However, I have a frequent problem: windows shuts itself down (not a powerdown, or BSOD - it shuts down and reboots) suddenly in the middle of my work.

the error signature displayed is:

BCCode / BCP1 / BCP4 followed by a code (this varies)
SP 2_0
OSVER: 5_1_2600
Product 768_1

The problem has been occurring more frequently recently, especially after using the zip drive for backing up files, working on excel spreadsheets, and working on documents on the slave machine.

I thought it might be an overheat problem, so we installed a new case fan and cleaned out the dust from the case to help cooling, and we have also upgraded the memory to a single 512mb dimm as it was a bit slow before and we work a lot with large jpegs. I also replaced the HP Cd-rw drive as it died with a Lite-On unit that works well.

A possibly related problem is that while i used to regularly defrag, at the moment the windows defragmenter freezes at 25% in analyse mode.

Because the Office and WinXP software came preinstalled, I do not have the CDs so a reformat is not really feasible (cost of Office + XP!)

Does anyone have any suggestions as to what might be causing these shutdowns, or a way I could find out myself by isolating the error?

Thanks,

KW