512Mb isnt a great deal but XP should run with it...
click start button, run, type in taskmgr and hit enter
this brings up task manager
click the processes tab
click the CPU column title to sort by cpu usage
with nothing else running (or running but not working hard) you should see System Idle Process as the highest ~ 99
If there is anything else using up lots of CPU you can click it in the Image Name column and click the End Process button at the bottom.
Click Yes to confirm the termination of the process (don't terminate explorer.exe as this is the process for the desktop/menu bar as well as Windows Explorer windows... (its not the end of the world as you can restart the process in applications tab by clicking the New Task button and typing explorer)
If you do have a process that is hogging CPU you can search for what it might be in google...
hope that might help a little