Virtual PC Low Disk Space

I am running Virtual PC on a Windows XP machine that has currently 4 VirtPC's on the machine. Recently after 1 1/2 years of running these, I am getting Low Disk Space Error reading on the machine. The Host machine has the bare minimum of required applications on it such as Office, so I do not think that removing software will help, has anyone else had this issue and how do you fix it?