I haven't tried this yet - I just reinstalled my Windows and am about to put Linux on it also. As of Vista, the bootup process for Windows has changed drastically and you could certainly end up with a non bootable system. I would seriously do your homework before you start this. Definitely backup your data first.
If you're willing to wait until tomorrow, I should be able to post any problems I run into tonight when I do it myself. I'm not using Debian though, I'm planning on installing Backtrack, UbuntuSUSE as well as UbuntuStudio.
Anyways if you decide to not wait, then I would at least get to know this page first:
http://support.microsoft.com/kb/933168. If I run into any problems, I'm expecting it to be from this. Also, you may need to get to understand the Diskpart program as well.
You can modify the size of your hard drive partitions using the Computer Management tool in Windows, no need for Partition Magic. Just defrag your hard drive first before you modify the parititions, that will allow you to get the most out of your drive when parting it, otherwise you could be seriously limited in the size of the new partition...
Good luck...