is it showing you any error messages? what brand and speed memory are you using? cheap memory will cause you problems indefinately. buy the best, every time. i prefer corsair brand, some people like others. i like corsair as they offer top notch products, great price and offer a lifetime waranty.
first, check the system logs and see what is causing your reboots. click Start>Settings>Administrative Tools>Event Viewer. Look at the system log, it may show you the possible hardware issue or check the application log to see if its a software issue, or possibly both.
on 2000 and XP, the system will automatically reboot if it cannot recover from the problem. check your settings.....click Start>Settings>Control Panel>System .. Then goto your advanced tab and click on "Startup and Recovery" Uncheck automatically reboot It might possibly show you the error message insted of rebooting.
hope some of this helps.