This might not be the best way (others more skilled than me can also answer here - but I'm usually quite reasonable in finding Work Arounds).
OK this could take a bit of time but this will I GUARANTEE 100% work.
1) Backup any USER data from your Windows 7 installation first.
2) Download GPARTED abd burn an ISO GParted -- Download
3) Boot it, delete all the partitions on it and create for your OS'es 2 PRIMARY NTFS partitions for Windows 7 (say 40 GB) and W2K8 server (say 30 GB) .
4) Now create and DATA partitions you need so your WHOLE DISK is partitioned and formatted. If there is ANY unpartitioned space Windows 7 / W2K8 server will create that wretched 100 MB system partition -- it's not the SIZE that's a problem but it's inconvenient - especially when you want SINGLE PARTITION IMAGE BACKUPS.
5) Then just install the OS'es -- I'd install Windows 7 FIRST BTW.
6) Good to go. You'll probably see a boot partition but that's unavoidable if you want to boot more than one OS.
W2K8 server will need a smaller partition than Windows 7 -- you presumably won't have too many Native applications running on a server.
However if you just want to TEST W2K8 server it's quite helpful to make it behave a bit more like a workstation -- you won't need all the stuff a server puts out when being used in a corporate environment such as Start up / shutdown logs etc. Convert your Windows Server 2008 to a Workstation!
Not sure why you would want to create a Dual Booting VIRTUAL MACHINE however - rather goes against the point of having a Virtual machine in the first place
Note my suggestion was for running on a REAL not a VIRTUAL Machine.