If you don't have a Windows XP CD, you can make bootdisk to boot Windows XP
Go to; http://www.bootdisk.com/bootdisk.htm and download "xpquick" to make floppy, USB or CD bootdisk. If the bootdisk doesn't boot, configure the boot device as first boot device the BIOS's Boot Menu. Pressing the Delete key right after starting the computer is the usual way to access the BIOS but if it doesn't work, look at the article for other keys you can use.
Once you enter Windows XP you can copy the three files in the bootdisk to C: (system drive) to renew the boot sector.