You shouldn't need to get a new copy of Windows XP. If you actually have to reinstall, just use your Windows CD or the recovery CDs that came with your computer.

"Operating System not found" would be an error message from your BIOS, meaning it can't find the initial startup files for Windows. This could be caused by many things. Do you remember what the blue screen said? Or what you did (if anything) to initiate it?

If you have another computer on which you can burn CDs, I would recommend making a KNOPPIX CD, which is a fully functional CD version of Linux (a "Live CD"). Booting from this you can probably check that your hard drive data is intact and potentially retrieve files that you want to save.