Problem:
The 30GB C: drive was running out of free space on my Windows 7 VM guest on VirtualBox v4.2.16.
Solution:
1. Shutdown VM.
2. Extend it to 45GB:
[text]$ cd /home/user/.VirtualBox/HardDisks
$ VBoxManage modifyhd windows7.vdi –resize 46080
0%…10%…20%…30%…40%…50%…60%…70%…80%…90%…100%[/text]
3. Extend partition size in Windows 7
a. Start the VM
b. Run diskmgmt.msc > right-click C: > Extend Volume...