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.