Compacting vhd file-hyper v 2008 R2

Hello,

I'm using Hyper-v 2008 R2. The vhd files are growing in size, although the data on the disk is not as half as the vhd shows.

I have tried defragmenting the disk within the virual machine and from the physical host but the drive is not shrinking.

how can i shrink the vhd file? the file is growing bigger than the disk size allocated to it and causing the disk to get out of space which is pausing the virtual machine.

Thanks