I got the message "a disk read error occurred, press ctrl, alt, del to restart"

I need to get this fixed. Is there a way to do it without losing all the data on my hard-drive...programs and such? Any tips are greatly appreciated.

I have a Dell Dimension 4600

I upgraded my RAM about a month ago, left my computer on in the basement and didn't check on it for a few days. When I finally did it was off and I got this message when I tried to turn it on.