Windows 2008 is the same as Vista in that is will 'reserve' it by using Superfetch to prepopulate it with what it thinks you will need, but that really isn't meaningful. Check
http://www.codinghorror.com/blog/200...my-memory.html for more details.
What you need to do is use Task Manager see
what process is using that memory. Also, use perfmon see if it is paging a ton, since that will really slow a server down, and likely indicate that the RAM is really being used rather than reserved by Superfetch. Tip: don't run it on the server, either locally or through RDP, but rather do it remotely by right click --> Connect to another computer, as it will inflame an already bad situation if you are logged into a desktop session (since it uses more RAM).
If I had to randomly guess, I would think SQL is the most likely culprit, which means you can simply try restricting its RAM usage, because I believe the default install will chew up as much RAM as you have available.