Compact Virtual Disks

I have searched the web and this forum for an answer, but I have not been able to find one so far...

Does anyone know of a way to compact virtual disks?  I have a virtual drive that is 100GB but only has about 8GB of actual data.  When I created the disk I specified that the disk NOT be split into 2GB chunks and that it not be preassigned (can't remember exact spec, but disk starts small and grows as needed, now I want it to shrink back when space is available).

I have tried defragging and it doesn't seem to change the size of the vmdk file.  BTW, I am running a Red Hat 4 Enterprise host with VMWare Virtual Server and M$ guest.

Thanks,

Ace