An easy  way to expand your disk without downtime:

1) download extpart.exe from the website :

2) put it somewhere on your network

3) expand your disk config in virtual center with e.g. 2G

4) log on to the VM, start command prompt and go the location where you’ve put the extpart.exe

5) extpart c: 2000

In Windows 2008 this it is not longer needed. Just start disk management in Computer management and choose rescan disks. After that choose : extend disk and ready…