well if the primary drive that's booting is the xp drive, check out
bootpart, which will let you use ntldr(windows bootmanager) to hand off the boot to your linux bootloader.
I suspect though(and I hope someone will correct me if I'm wrong), that if you installed fedora when its drive was in the primary master position, you're going to run into problems trying to use it from the primary slave postition. one example is that your fstab will probably try to mount /dev/hda1 as / , which would be correct if the drive was in the primary master position, but now (in this hypothetical situation) it's in primary slave position and /dev/hdb1 should be mounted as / , not /dev/hda1, but that isn't how it will be setup if you installed fedora when the drive was primary master. basically, you won't be able to boot without altering at least your fstab and your boot loader's config. It's possible, but I wouldn't want to deal with it, and it's likely there's other little bits that won't be setup correctly too.
You'd be much better off installing both drives, installing windows to the first, and then installing fedora to the second, at least IMHO.
