I've had this before.
The problem is most likely that the partition numbering is different from what your boot.ini expects.
You have a line where it says something like:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
Chances are that after creating the new partition it is now "...partition(0)" - and therefore it can't find ntldr.exe on that partition. Change boot.ini to reflect your new settings, and it should boot.