Windows XP: ''Serious error'' msg and question re ''registry cleaners"

''Serious error'' msg and question re ''registry cleaners"

We have a 2-year-old Dell Dimensions running WinXP Home Second Edition which was reinstalled about six months ago. There have now been two occasions where the computer has crashed and when rebooted, has shown the message ''Your computer has recovered from a serious error.'' The first time was after I ran a system scan with Norton Antivirus. Microsoft returned a web message that said that the problem was probably caused by Alfa File Protector (AFPAnsi.sys). I tried to find out more, but really never figured out what to do. The second time was this morning, after the machine had been left on overnight with no activity. It was the same situation: Completely dark monitor screen, no response except from shutting down power and restarting. This time, Microsoft Error Reporting returned a message saying they didn't know what had happened. What is the best way to proceed? Is there some fatal flaw in the hardware or operating system? Would a ''registry cleaner'' help? (which leads me to my second question): There is a bewildering variety of freeware, shareware and buy-only programs out there that purport to analyze, clean, and tune up the system registry. Are any of them worthwhile? How do you choose?