We use a lot of Virtual Machines at work (mostly using VMWare) and unfortunately they are not always configured with sufficient disk space. If you need to increase the space allocated to an existing drive then check out the following article for step-by-step instructions:

• HOW-TO: Increase the size of an existing VMWare Hard Disk (VMDK)

The open source disk partitioning software recommended in the article also seems to work a treat once the space has been increased.