Buy a new HD. But in the meantime, try using Single User Mode to have the computer check and repair disk corruption.
To get into Single User, at the boot sound, hold "cmd" and "s" down until the screen goes black. When you get to the prompt and white text has stopped scrolling by, type this (without the quotes): "/sbin/fsck -fy"
This will run a check disk very similar to Disk Utility but will be more thorough. That may allow you to use the computer enough for you to get your data off of the HD.
At this point, getting your information off the HD should be your first goal. Sounds like your harddrive has one foot in the grave.
Tonka