I ran DISKPART on a virtual hard drive to increase the maximum capacity. However, I appear to have made it too large for Virtual PC (win7) to run. I shrank the volume back down, but the maximum available size is still what I set it to. I can't seem to find a way to reduce the capacity of the disk. Running the diskpart SHRINK command just shrank the volume, but not the "physical" space.
For anyone who is interested, what I ended up doing was using the VHD Resizer tool (seen here), which worked like a charm.