If you are not overclocking and the only thing you did was re-format and start over...then my friend you have a security leak. That is what happens when you don't have Windows updated. There are tons of security updates for Windows XP, USE THEM!!!
Manual patching theses holes is very much impossible for the normal or even experience non-programmers. There is just so many items to deal with and I don't know where to start.
If you don't want to update Windows, then you need to talk to a hacker. They have the best knowledge how to break in and find security leaks, they might be able to help you to patch them.
In other words, pay the $99 bucks to get yourself a registered copy of Windows or learn to code and make the same security patch that Microsoft with lots of expert programmers to patch them for you. That's pretty good for $99 bucks.
It's not that I do everything by the book, but I love to use my computer so usually pay a little bit of money for hours, days, years of fun on my computer.
Sorry, that's not the answer you wanted but I at least gave you options. And remember you can always scrap Microsoft and go with Linux.