You could try a couple of things.
Here is a link to an NTLDR fix:http://tinyempire.com/notes/ntldrismissing.htm
You will need to burn a CD or floppy and then once on, you can determine what needs to be done to fix it.
Or, have you treid a Repair Install, using the OEM CD? After it boots up, it will ask if you want to repair using recovery console or install. Pick install.
On the next screen it should give you the choice of repairing the existing OS.
hope this helps.