|Adding more memory wont do a thing for a full hard disk.|
Do you get this message during boot-up or when loading the applications you use?
We need to know whether it is an Operating system error or an application error.
The start up disk will be the disk the Operating System and/or the application is installed on. This is usually drive C: You need to see how much space you have on that disk?
Running the disk clean-up Utility will free up some space by getting rid of a lot of the temporary files that windows creates and forgets to erase.