On Windows XP, most hard freezes are video drivers related or RAM fault related.
Try upgrading your video drivers. You can have them on www.nvidia.com. There are NO charges for the drivers.
Go to the Support page.
Click the "Download drivers" button.
Use Option 2: Automatically find drivers for my NVIDIA products.
This will automaticaly detect your card and operating system and start the download for the appropriate drivers.
Double click the downloaded file to start the drivers update. Restart the computer when prompted.
If that don't solve your problem, you may have faulty RAM.
Do a web search for some RAM testing application and run it. It can take over an hour to complete the full testing suite.
If at any point you get any error message, you can stop the tests: You found some defective RAM module that need to be replaced.
You can go on http://www.crucial.com/ to check what kind and amount of RAM your computer support.
You should think about abandoning Windows XP as it reatched it's end of life point. Windows 7 is stronger and more stable. Check on the Microsoft site to see if your computer is good enough.