hi befuddled,

I would try this before buying a new copy of Win XP-

With windows XP you can try to do a system restore-
where you go to- start /programs/ accessories/system tools/ system restore -
and click that and it will give you prompts /choices to roll the operating system back to a date where you think it was working well.

It's pretty easy to do and explains the steps as you go though it.
and if it doesn't help you can reset it or try a new date.

* my XP CD is 4 yrs old and I reinstalled it twice and also did a fresh install on my new hard drive this spring. My old hard drive had crashed multi times - had bugs and viruses.
I did have to update all the patches and security stuff from the Microsoft website but it was pretty easy to do. I just clicked on the Windows Update and it found everything for me.

If those fixes don't help you might need to get a new hard drive not a new copy of XP.

Do you have some anti virus, ad ware and spyware running all the time or even just run them once a week or so?

On our computer & technology forum we have some good links for free software for those kinds of things.
lots of tips and suggestions too.