RAM upgrade will help as above posts indicate, but unless you're doing a whole lot of multi-tasking, you may not notice a huge jump in performance.
If your XP is in it's original installed status, sometimes a re-install of the OS helps tremendously.
Another quick partial fix would be to clean up your dead end files & accumulated rubbish.
CCleaner: http://www.filehippo.com/download_ccleaner/
Download; create a 'Restore Point'; install CCleaner.
Then, (using the default settings):
On the main CC window, the 'paintbrush' tab is for the Windows components that you can clean; (in the 'Advanced' heading, check 'Hotfix uninstallers' to clear these files) & tick the bottom right box (Run Cleaner) and it will delete (or overwrite option) those files.
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Same procedure for the 'applications' tab.
Next, down from the' paintbrush' is the 'Registry' icon (like a Rubik's Cube); first tick 'Scan for issues' button on the bottom; wait for the process to run.
It'll show a long list of entries for the first couple of cleanings; (Back up your first couple of registry cleanings, and if your unit gets ungovernable, restore those files. I've never had to do it, so I can't tell you how. After that the cleanings will be for empty folders or other junk so backup likely won't be necessary).
When ready, tick the 'Fix Selected entries' and CC will take care of those.
For your 1st few uses, have checked the 'show prompt to back up registry files'; open CC> Options> Advanced> check the 'show prompt...' box; then tick the 'yes' or confirm to back up the registry. (It gets saved to the 'c' drive, as 'ccxxxxxx')
Also, run the 'disk check' which will look for physical problems on Hard Drive disk sectors.
For XP do this: Start> my computer> right click 'c' drive> properties> tools> error check: here check both boxes, then OK. The next time you cold boot your system, this process will run (maybe 30-60 minutes) fixing disk errors(if any).
Many other things can be causing the slow performance, but the above ideas will get you started.