how big is your power supply?

You're probably getting blue screens, but XP's default setting is to auto reboot on a blue screen. To turn it off so that you can actually see the error you're getting, right click my computer and click properties. Go to the advanced tab and click settings under startup and recovery. Uncheck the box labeled automatically restart. Won't solve your reboot problem, but at least you'll be able to troubleshoot it now.

Or, just check the Event Viewer to see what's causing your system to self destruct.

Here is the error im getting , can anyone shed some light in this? Cheere in advance
Bob
 
 
 
Event Type: Error
Event Source: System Error
Event Category: (102)
Event ID: 1003
Date: 09/11/2001
Time: 17:18:40
User: N/A
Computer: BOB
Description:
Error code 0000000a, parameter1 00000004, parameter2 00000002, parameter3 00000000, parameter4 804f1a9e.
 
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Data:
0000: 74737953 45206d65 726f7272 72452020
0010: 20726f72 65646f63 30303020 30303030
0020: 50202061 6d617261 72657465 30302073
0030: 30303030 202c3430 30303030 32303030
0040: 3030202c 30303030 202c3030 66343038
0050: 65396131

This used to happen to me too (reboots for no reason), It ended up being that my Nvidia drivers were bad (the ones that came with XP) so I upgraded them to the newest ones and everything's cool. So if you have any drivers that you can update, update them.