Right click on 'My Computer' then go scroll down to 'Manage'.
Then goto 'System Tools' and then 'Event Viewer'.
Inside 'Event Viewer' you will have the options of 'Applications', 'Security', and ' System'. Go through each of those to determine whether or not there are any "Red Xs" or "Warning Signs" in the any of those areas. If there are let me know and we can figure it out.
If there isn't anything in the event viewer, it may be a memory issue. Download and run 'Memtest86' from here.
Burn that to a floppy and reboot. Then let it run through the patterns to determine if it is a memory error. Random reboots and sporadic reboots are most likely the product of bad RAM or memory bearing hardware. For example: video card or RAM.
First, read into the Event Viewer and tell me if you find anything that draws your attention.