Windows XP Pro Error Message

I am getting an error message whenever I boot my PC. Operating System: Windows XP Pro Error Message: Isass.exe- application error
Application failed to initialize properly (0xc0000006). Click OK to close.

When you click OK, a blank blue screen appears.

I checked the Microsoft web site and they don’t even have a topic about this. I checked some websites and the solution is to re-install the OS.

I need to know what this file is and what causes this error before re-installing the OS orif there are other options besides from re-installation.

Help?

Thanks.