|Your system is possible producing a BSOD which you cannot see because the system is set to auto restart on system error.|
The following is extracted from http://www.microsoft.com/windowsxp/...
1. Right-click My Computer, and then click Properties.
2. Click the Advanced tab.
3. Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.
4. Clear the Automatically restart check box, and click OK the necessary number of times.
5. Restart your computer for the settings to take effect.
Next time your system crashes you should see a BSOD, please post the codes here.