I would add more physical memory 256mb is not really enough for XP, you really need 512mb +.
However your PC hanging could be due to many problems.
First use your hdd manufactuers diagnostic tool to see if the hdd is faulty. See this post for the full list
Run chkdsk on your drive.
Defrag the drive
Scan for virus's/malware.
Update your drivers for graphics cards etc.... (although you may need to downgrade your drivers to a early version as the latest ones may be causing the problem).
Have you recently added any new drivers/installed software?? If so uninstall and see if that cures the problem.