Best Registry/Windows Cleaner and Fixer?

Quick info, running XP Pro on a business network of 5 PC's, all PC's have original install of XP with all current updates... I can not format and reinstall any of the HDDs on the PC's because I do not have the Install file for the POS program that is installed on all of them.
Original Install was 4 years ago. Since then most of them have all new hardware (mobo, processor, ram)... have uninstalled the drivers from old hardware and installed the new drivers.

Problems now is, the server and the individual PC's sometime looses connection with one another... sometimes some of them decide get to the Windows Login screen then restart itself when you try to log-on... just various stupid things.

I'm figureing a Registry fixer/cleaner or some program of that nature will help cure problems?

I currently have ZoneAlarm Extreme Security running on one PC with all it's updates and used it's "PC Tune Up" program... it seems problems with the registry and corrected them but it didnt seem to "fix" any other the anoyences.

Thoughts and what I should do or another program to try out?

Thanks all!