--Another IT type here.
Backing up your files in the manner recommended is most excellent advice.
I would hold off on buying a new OS until I was sure that this was not a hardware problem. The symptoms you describe could be caused by corruption of your OS or malware, but they are also quite common when a system's hardware is failing. I see this on almost a daily basis on systems that have reached the 5 to 6 year range in age, especially with entry level Dells, Compaqs, HPs and eMachines that use "Bottom Feeder" motherboards.
Hopefully you have an XP CD or a system restore disk from your PC manufacturer. Backup your data in the manner described above. Be thorough. It's easy to forget something.
Assuming that rolling your system back to a previous date will not fix things, the next step to try would be a repair install using either the repair function on the XP CD (Clean boot from the CD) or the System Resore function from disk provided by your PC manufacturer.
Either of these options will work if your PC is healthy hardwarewise. If they don't, then it's time to think about repair/replacement of the PC itself.