It sounds like your hard drive is too full, and it is imperative that you free up some space or risk a crash and losing everything.
As soon as you can, get a large external fire wire drive and move what you can onto it. Good candidates would be large video files, large music files, maybe iPhoto if you have a lot of photos. There could even be some rogue file which is taking up lots of space. Once something has been moved onto the external drive, delete it from your internal drive and empty the trash. You won't recover the space until you empty the trash.
A good way to see what is taking up space would be to download one of the following:
WhatSize:
http://www.id-design.com/software/whatsize/
Disk Inventory X
http://www.derlien.com
Both used to be free, but I've heard there is a small charge for WhatSize now.
It's good practice to keep 15%-20% of your hard drive free if you can.
To check, open Disk Utility (in the Utilities folder in your Applications folder) and click on "Macintosh HD". Then look down at the bottom right bold faced table and see what it says about "Capacity", "Available", and "Used".
While in Disk Utility, click on you hard drive by brand name and check to see that the S.M.A.R.T. status is verified. If it is not and there is a message in red letters, please report back with the message.
But put a priority on getting everything important backed up to an external drive.
Good luck.