Sounds like you need to delete some files off of your hard drive. Computers need some space on the hard drive that isn’t taken up with stuff you want to keep so it can use it to write things temporarily.
Are you using Windows? The very first thing to try is emptying the recycling bin. Things you “delete” go there first, as a way to give you a chance to change your mind. They’re not really gone off the hard drive until you empty the recycling bin. Right click on it and choose “empty.”
If you’re on a Mac, the same thing applies but it’s called something different. Trash bin, I think. (Windows stole the idea from Apple and then one-upped them by calling it recycling instead of trash. Heh.)
Then if that doesn’t help, you’ll need to go through your hard drive and delete old stuff you don’t want anymore. Music and video files are the biggest ones, so getting rid of them will help more than getting rid of Word documents. Pictures are often really big too. Don’t forget to empty the recycling when you’re done.
If you can’t find enough stuff to delete, you’ll need to move some things to a different place. You could burn them to CDs if you have a CD burner, or get an external hard drive and move some stuff to that.
But to be sure, it would be helpful to know the exact wording of the error message and when you get it. Does it happen when you start your computer up? When you try to save a file? When you open a certain application? Just randomly?