As you know we recommend setting the VHD for your SBS deployment to be a fixed disk. By default Hyper-V will create a Dynamically expanding disk. Which does have its advantages like being faster to create, and not taking up white space on your hard disk drive. However they do have some disadvantages, namely that you can over subscribe your hard disk with multiple VHDs, the white space can become fragments, and they can cause VHD fragmentation on you hard disk. In order to ensure optimal performance you want a standard fixed disk VHD.
Creating one is simple;
There you have it. A new virtual machine ready to deploy SBS 2008 with a fixed sized virtual hard disk.