You can do it from the boot.ini file also:
Just Click Start, click Run, type sysdm.cpl, and then click OK.
On the Advanced tab, click Settings under Startup and Recovery.
Under System Startup, click Edit.
There must be 2 lines on your file similar to this:
default=multi(0)disk(1)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(1)rdisk(0)partition(1)\WINDOWS="Windows XP Professional" /fastdetect
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows 2000 Professional" /fastdetect
At the command prompt, type:
bootcfg /default /ID#
Where # specifies the boot entry id in the operating systems section of the BOOT.INI file to be made the default operating system
Savvy!