Blank screen appears frequently

I have a Pentium III at 300Mhz with 64 Mg RAM.

The screen goes blank once in a while and then I have to reboot the PC which means I loose Internet connection and/or loose any thing I was working on at the time. Why is it doing this? Please HELP.
I have already done a Scandisk and defrag the drive.

Thank you

Sam Premji