Machine Freezes Frequently

I have an Xcel 2000 M748MR system board. I have Winnt4 (service pack 3) on the machine. When working on the machine it often freezes up on me. The only way to get it back working is to turn it of and then back on again. The machine always freezes when the restart option under shut down is selected. It will be greatly appreciated if someone could help me resolving the matter.