You have the following options to repair the boot sector files.
1) Use your Windows XP CD to repair the Boot sector through the Recovery
Console with the "FIXMBR" command, followed by the "FIXBOOT" command. See
how to in the TechRepublic.com article
2) From another computer, go to the bootdisk website and download
"xpquick.exe" to the hard drive, format a 3 1/2" diskete, and execute the
"xpquick.exe" file to make a bootdisk and start your computer with the
diskette in the drive, then copy the files from the diskette to the boot
sector (to C:\)
3) If your computer doesn't have a diskette drive, you can burn the bootdisk
files on a CD and start the computer with the CD in the drive
http://www.bootdisk.com/bootdisk.htm
10 things you can do when Windows XP won't boot
http://articles.techrepublic.com.com/5100-10878_11-6031733.html