This symptom is caused by Viruses. It is advisable to run a full system scan. Be sure you update the anti-virus software definitions. The following viruses terminate any of the following processes: REGEDIT.EXE and TASKMGR.EXE
For additional help see www.dougknox.com, Win XP Utilities, Create Emergency Copies of Critical XP System Utilities. This small VB Program will create backup, usable copies of Task Manger, Regedit and MSConfig (named Taskmgr1.exe, Regedit.com and MSConfig1.exe) in a new folder C:\EmergencyUtil. Many virus programs will intercept these programs, based on their original file name. The modified file names, allow them to be run. Open Windows Explorer to C:\EmergencyUtil and double click the application you need. The next revision will allow you to browse for the folder you want to place the backups in.
Additionally, see the Win XP Utilities section for Startup Programs Tracker. This small utility scans your system for startup programs and running processes. It also allows you to create a log file that can be copied and pasted into a newsgroup post. The contents of the program window are also copied to the Windows Clipboard, automatically. For replies to newsgroup posts, do NOT include the Running Services, unless its absolutely necessary.