Storage

There is a number, approaching infinity, of things which could be wrong. You'll have to be a bit more specific.

please include more details

without a CD to load the ISO on, you're SOL...besides, everyone knows
Windows XP SP3 can't be repaired since it isn't broke...there is a major
problem, BTMATKB, only you can resolve this one.

I think you have an ISO on an existing OS harddrive but no CD (aka netbook?) Here is a link to installing WinXP using a USB drive. Note that you should run a utility like keyfinder to get the existing OS CD-Key before reinstalling WinXP (or look at OEM tag on bottom of theoretical Netbook)

http://wiki.eeeuser.com/windowsxpusb
http://www.magicaljellybean.com/keyfinder/