I don't have a backup disk, is there anyway I can reinstall window without one ?

Simply put NOPE, without any sort of Windows disc your up the creak without a paddle.
Sorry :(:(

If you lost the disk - and it was registered with Microsoft - you can probaby get a replacement disk.

... was it registered to you?

You can get the software disc off the internet all you need is the cd-key
If you have that do on a bit turent site and download it
The one problem is that it might not recodnize your cd key as a valid one

By backup cd you mean the windows xp cd that you can use to start from a fresh version of windows right?

Getting an illegal software key from a bit torrent site will upset your PC.Plus you won't know what hidden "code" is in the download.Dont do it please.