Try here to see what ram you have
http://www.crucial.com/systemscanner/
Then ctrl alt delete. This will bring up windows process manager. Click on the processes tab. This will show what each process is using.
If you are running a lot of programmes that are using your memory up there is the problem.
There are other settings in XP which can help but basically if your ram isn't big enough to run stuff this will always slow the system down.
If by memory you mean hard drive space.
As others have said get a USB hard drive.
They're not too expensive. Then just click and paste stuff like movies and other big files to it.
that will free up hard drive for the system.
Perhaps if you could add what size system you have that would help (hard drive, RAM, operating system, processor type and size.)