So I plan to reformat soon. I have all backups made, and im ready to reformat. One slight problem. I don't have my windows disk...... i have the product key and all, just no disk. Now im wondering, could i burn an ISO of the disk to a cd and then run that to install it? Or will i have to get an actual disk...

That should work

I don't think it will work. The key has to sync up with the copy of Windows XP. It MIGHT work if the iso you download is the exact version of XP that you had before, but otherwise it will come up invalid code if you do not get a cracked copy.

It would work technically yeah, I torrented a copy of XP professional once -cough cough- , but you need a CD key that matches the CD. It's a way to stop pirating like what Jeff is saying. I think if you called MS they might send you a copy of windows XP providing you give them your product key, maybe? I think you can, just tell them you lost it.

For the love of...

Just download a cracked Windows XP torrent. Use whatever key it comes with. It doesn't matter. Microsoft isn't going to come breathing down your neck. You have a legit key and that's all that matters if the FBI comes knocking on your door.

but you need a CD key that matches the CD.

Thats not true, a windows XP home product key should work with any windows XP home installation regardless of what CD it was installed with.
The only problem I could see the OP running into is if the product key he has is for a manufacturer OEM license.

Mama Luigi is right though. You DO technically own the right to use XP so if you get a cracked version there's no ethical dilemma if you cared at all to begin with.