This was a serious pain in the arse. The first time I installed Linux I was getting "disk geometry errors" with Partition Magic when I went back into Windows. I had decided to let Linux create a partition for itself rather than prepare one in advance and this proved to be a mistake.
I've learned the hard way that the best way to install multiple OS's in a Windows-first setup is to simply create additional *Windows* partitions on the drive first and *don't* try to prepare them for an OS (using Partition Magic or whatever). Then select the appropriate partition during the installation of your OS.