How to Compact the VMDKs with window guest

Dear ,

I have a problem in manage vmware system storage, i use ESX 5.0 host and IBM storage attact directly to it and was mounted, my VM was created with 200GB HDD (thin provisioning). Some recent days, when i browse datastore, in the location of that VM, the .vmdk file is alway ~ 200GB but when login to the VM, the real data was just ~50GB.

I read this thread http://communities.vmware.com/thread/301819

but my case is window 2008 guest OS, how can i reduce my VM .vmdk file ? and i want to deeply understand how does vmware works ?

Thank you very much