Someone posted a comment on the Activation Rearming guide asking how to do this, it's fairly simple but here ya go.Assumptions:
I'm assuming you're on Server 2008 R1, you're using the default (Administrator) user and you've installed on an NTFS partition.
This can however be done on any Windows OS (from Windows XP to Server08 R2) and with any user with admin privileges as long as you adjust accordingly.
First, you must have a second partition other than C:\ - if you don't have one, you can either use a (a) make one when you're first installing Server08, (b) use a Linux LiveCD to create one, or (c) try using Server08's partition manager - I've tried (c) and failed with this so I'd recommend Linux assuming you've already installed Server08. You can get a really good LiveCD here
and it's only a 94mb download (yes it's a fully functional and very decent OS) - pop that on a CD, boot it up and mess around with your hard-drive all you like.
Once you've got two separate partitions - C:\ and lets call the other one G:\
Now every time you reinstall, you just leave G:\ and everything on it intact and install on to C:\ - repeat steps 1-4 and you're sorted.