You must change few lines in boot.ini file.
You can do it like this:
Open windows explorer and go to root of your sistem disk (usually c

, now
locate file boot.ini and double click it. If you don't see this file, than
you must change folder options, by clicking tools/folder options/view/ than
uncheck hide protected operating system files and select show hidden files
and folders.
Now, your boot.ini file looks probablly 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(1)partition(1)\WINDOWS="Microsoft Windows XP
Professional" /fastdetect
Pay attention to the lines below [operating systems]. Here are listed all
operating systems, that are installed on your comp. Pay attenition to the
numbers between ( ) after the words rdisk and partition. This number shows
the location of your system. If it says (0) after rdisk, than this is
usually first disk, if it says (2) than this is second disk and so on. Same
applies to the partition. Now you must just delete the right line. In this
case, it would be the last line. Save the boot.ini and restart the system.
Just be carefull when modifying this file, because if you do it wrong, your
system might not start.
Here is also the short way of deleteing second operating system from
boot.ini:
At the command prompt, type:
bootcfg /delete /ID#
Where # specifies the boot entry ID that you want to be deleted from the
operating systems section of the BOOT.INI file (e.g. 2 for the second
Operating system that is listed.)
You might also want to read this:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;289022
Good luck