Reformatting Windows XP without original CD

Weapon X

So basically a few weeks ago I got some generic trojans on my computer. I have Trend Micro Internet Security installed, and its been fairly reliable. Except that somehow this trojan got through via a download and Trend Micro didn't notice the file was infected. I've determined where I picked it up in the first place...a seemingly harmless program install (nothing pirated mind you). I seem to have gotten rid of them through help of the internet and a few virus scanning programs. But now my computer is acting strange and pitifully slow and I feel like I need to reformat as soon as I can before it shuts down permanently. I've had the computer for about 5 or so years now, and I have totally lost the original Windows XP Home install disc. I still have my original Product Key (its on a sticker on the computer itself).

My question is,
1) Would I be able to download and burn a Windows XP Home install through nefarious means and still be able to install using my initial Product Key?
2) Am I going to have to go to a friend and borrow a CD etc.
3) Do I even need to reformat at all? Aside from potential virus/trojans hidden in my computer, it would also be nice to have a clean sweep since I know this computer has a lot of useless crap on it.

Let me know if you need more information, thanks in advance.

blakfeld

Microsoft's official stance is "We don't care what disk you use, but it has to be a key licensed to you"

So, options 1 or 2 is the best bet, although if your willing to dick around in msconfig, you could probably clean up most everything, but a restore is always nice every so often

Crashtard

http://magicaljellybean.com/keyfinder/ That will help you find the product keys for everything installed on your machine.