I think you are confusing memory types here.
A computer has 2 main types of memory
1) REAL MEMORY (or RAM) - this is only used when the laptop is running. When the laptop is turned off it becomes empty.
2) HARD DISK MEMORY - this is where all your programs, personal files etc are stored. The information on this memory remains even when the computer is turned off.
My guess is that it is your HARD DISK memory is 3/4 full, so adding more RAM memory will not help this at all.
You could replace the HARD DISK with a larger hard disk, but that would mean a complete reinstall of Windows and all your programs.
As other have said, buy an external hard drive. This plugs in your USB port and is very easy to setup and use.
Here is an example of one.
http://www.maplin.co.uk/Module.aspx?ModuleNo=2 17655&source=1&DOY=2m5