Since you didnt say what OS you are using, if its XP, There is an option that causes xp to reboot on an error. Go to system properties (right click on My Computer on the desktop) click on the advanced tab, where you will find startup & recovery info, click on the settings button for it and in the new box that opens uncheck automatically reset. That way you may get some info on what the error is and we may be able to help.
Another thing i'd do is check for any virus. See
this resource thread and run at least 2-3 of them.