There will be problems ahead when you trying to install Windows on disk which already have 4 or more primary partitions which is in you case. Windows Installer will refuse to be installed to any partition apart from the first 4 primaries. And if you exceed 4 primary partitions you will be running into partition tablet syncing issues because you are essentially creating GUID/MBR hybrid, and there will be 2 partition tablets of different type co-exist on the drive. I know of a linux utility to resync the partitions tablet when its out of sync.
You can still can install Windows on this hard disk but you have to:
1. reduce the exist partitions to 3 or less which mean removing SL or Lion.
2. reinstall the SL or Lion you removed in step 1 afterward
3. install a debian linux too to resync the partition tablet when required:
Linux terminal:
gptsync /dev/sda
(may be a debian base e.g. ubuntu / LinuxMint live CD would be sufficient)
Perhaps separate drive is still the way to go and it much less complicated.
Good Luck