You need to open up C:\boot.ini in a text editor and delete the line that doesn't correspond to your default Windows installation.
The file should end up looking something like this, i.e. you need to have just one entry under [operating systems]:
Code:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional" /fastdetect