that means your bootloaders missing or corrupted (common and annoying problem), but you can fix it by following this guide
word for word Simple "ntldr is missing" fix with boot floppy, CD-R, or USB flash
also you could use you xp disk, go to recovery console and type:
bootcfg /rebuild
fixboot
fixmbr
(sometimes works)
but in my opinion its much quicker to whip out your winxp disk, start setup and select
repair existing installation when it prompts you