[reboot]

Just edit boot.ini in notepad.
It will look something like this:

[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
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS = "Microsoft Windows"

Make it look like this:
[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

In other words, just remove all the stuff relating to the second OS.
DO NOT change the numbers inside the brackets () !