Windows 2000 Rebooting

I have just bought a new Advent Pro PIII 1G. In order to save time setting up the computer exactly like the one i was previously using, i decided to just swap the harddisk drives over and hey presto – my new machine was setup in 5 minutes. It installed all the drivers itself and worked fine for about a week. Just recently, at least once a day it has decided to just reboot when ever it feels like it !!! There is no theory behind when it does it either. It can be when you press return after entering a URL in explorer or when you click the file menu in MS Word !!! Please help me…….