Firstly, Group Policy Editor is only available in Windows XP Professional. If you're using a different version, you won't be able to access it. Also, update to the latest Windows XP Service Pack 3. Try updating all your drivers for the system, and also run a thorough memory scan of the system. The random closing of the DOS window is really odd. It could be memory problems or just Windows. You should try a Windows repair instead of a reinstall. Viruses could be a problem. Try a different anti-virus or any of the free online virus scanners to be sure.