First of all, remember that Windows 2008 has an "especial" folder, called Winsxs, that can grow very big (usually 6-10 GB). I recomend a SO partition not less than 40 GB.
I'm supposing that the server is a real server hardware with two or more disks on raid 1 for the SO partition (standard build), if not, it will be more tricky to do the job.
Updated after reading your info
Check a BartPE or Windows PE Start CD with a partition software installed on it BartPE you can check the info. I remember have used it with a ghost installed to:
- Cloning the Disk out after defragmenting and checking all the options
- Resize the Virtual Disk
- Restore the Image to a less partition (but big enought to hold the Image and some free space)
Maybe this option fits your needs.
Please mantain us updated. We all have cheap dell servers elsewhere :)
What I have done in the past:
- Defrag the So partition with a defragmenter (windows one can be enough)
- Remove one disk from raid, in the worst case, you have a working SO in that disk
- Use Gparted, a Live ISO, run it from CD, load the Raid Drivers if necessary and reduce the SO partition to your needs.
- Start the SO, check and if all is OK, put the Disk on again to rebuild.
If anything goes wrong, shutdown, remove the online disk, put the disk backup, start the SO, check and put the disk to rebuild.
I like this system because you have an easy backup to restore without external software or media and in no-time.
Observation In case that you haven't raid, Gparted can do the same work, but you will need to make a Backup from the system in an extenal media. It can work on Virtual Machines too... A must have in my opinion.
Hope this helps.