You can copy a new one to the root directory. It should be similar to this:

Code:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptIn
You may need to modify the lines beginning "default=" and "multi"depending on what drive your XP installation is on.