What is so hard about reformatting for people? It takes about a day of your life and you can again use you PC to the best of it's ability. Or you can spend a bunch more money than the PC is worth upgrading the thing to make it faster. The only surefire way to get it up and going like it was new is to reformat. So what if you have to reinstall all your software and reset all your settings, I would rather do that than spend a couple hundred upgrading hardware in an attempt to make it faster.
Just get your Windows key using magical jellybean keyfinder, or look at the key on your tower. Get a copy of the version of Windows you have. If you have a key for XP Home, make sure you get a CD for XP Home. Back up all your data to CDs, DVDs, or an external or secondary HDD. Find any driver CDs you have, if you don't have them, download the drivers from the internet. Plug in the CD and restart the computer, it's pretty simple to reinstall Windows from there. When it asks for you to type in the 25 character key, find the key you had earlier and type it in. Once it's up and running install any drivers and then install all your software and you are all set with a speedier, less bogged down system; and it only took a day of your life rather than cash from your pocket. I dunno, which seems more logical?