Problem
A Virtual Hard Disk (VHD) is a disk image format used by Microsoft Virtual PC. VHD enables users to test software on different operating systems without installing a second hard disk or a new operating system. If you want to open VHD files, you could use a small vhd viewer to view and extract VHD contents.
Solution
WinImage is a disk imaging application and also an useful VHD viewer.
Tips
See also
How to Convert VHD to VMDK
How to Convert VMDK to VHD
How to Mount VHD Files in Windows 7
How to Boot from a VHD in Windows 7