Are all these on the same physical disk?
The only problem I could see would be if the G and H drives are on the same extended partition (unlikely) in which case you would have to do some messing around.
Otherwise it should be pretty trouble-free.
Good luck.
Mandrake will install a boot loader for you. Just stick it in the MBR of your first disk. That will give you the choice of Windows or Linux at boot time and changing the default is just a matter of editing one line in a config file and (if you use lilo) running a single command.