Hi Bob,
I have never done what you are suggesting - but it is possible.
Have you given any thought as to how you will know which OS you are working with once booted up .....or are you keeping the second option "just in case"?
You could always clone the HDD to a second disk.
That way - if the Hard Disk dies - you can just pop in the spare.
Do you have the software to clone the primary partition?
The BIOS may sort it automatically and give you the option.
Usually the screen will wait for a response and if there is none - it will load the OS on the primary partition.
You may need to alter the Boot.ini file if the second OS is not detected.
It would look something like this when you add the second OS:
[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 in Partition 1" /fastdetect
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows XP Professional in Partition 2" /fastdetect