Are they on the same drive or partition?
If they are on seperate partitions you should be able to reformat the non-working OS partition from the working OS in disk management. You may also need to edit the boot.ini file. Here's some info on that:
<A HREF="http://asia.cnet.com/itmanager/tech/0,39006407,39140825,00.htm" target="_new">http://asia.cnet.com/itmanager/tech/0,39006407,39140825,00.htm</A>
<A HREF="http://www.jsiinc.com/SUBI/tip4300/rh4397.htm" target="_new">http://www.jsiinc.com/SUBI/tip4300/rh4397.htm</A>
....WW (5.0)