Help me reduce VHD file size when deleting data in OS.

I use Hyper-V on Server 2008 64bit installation to create a CentOS 5.8 VPS has the capacity 100GB. After a period of use of disk space used up nearly all 100GB. I have deleted a VPS data in this section but I checked VHD file size not reduced. 

The way you can help me do this? I thank