If you are seeing random system crashes or periodical restarts, it may help you to know what the most common reasons for Windows XP to become unstable and usually lead to what we affectionately call the BSOD (blue screen of death). They are listed in random order off the top of my head:
Bad Memory Module: Having a bad memory (RAM) chip will cause these issues. To troubleshoot this, you will need to either swap out a module for a known good module one at a time until you find the bad module. Or, remove one module at a time and reboot your system. Repeat this process as necessary until you find the bad module. I can not begin to tell you how many times I've fixed an issue just by removing the bad RAM chip.
Sound Card Driver: It is fairly common for a driver to cause this type of issue. Sometimes they become corrupt for no reason or even become out-dated. The sound card driver is not the only driver that can cause these issues, but it is the most common and should be the first card to focus on. Just simply visit the manufactures web site and reinstall the correct driver for your sound card.
Overheating: Once your systems processor reaches a certain temperature, the system will shut itself down. The most common reason for overheating is a dirty fan not being able to spin fast enough to cool the processor. Or, a fan has just gone bad. Once you've gained access to the inside of your case, look and listen for a faulty fan. If they all appear to be working, it may not be a bad idea to go ahead and clean them while you are in there.
Failing Power Supply: A power supply that isn't providing enough power to create a stable environment will cause this type of issue. It would seem if your computer turns on and boots up that the power supply is working, but in fact, it could be failing. This can be a tricky one to troubleshoot if you do not have the proper equipment. Without the proper test equipment, you are forced to either replace the power supply with a known good one or replace it with a new one. On the other hand.....if the PC does not boot at all...this maybe your first sign that the power supply is dead making your decision easier.
And lastly....a bad USB Hub: This one is pretty simple to troubleshoot. Just simply remove the hub from your system to see if the problem is resolved. This is not nearly as common as the scenarios listed above, but I have seen it happen a few times.