I use VirtualBox OSE for my virtual machines and really love it. I tried to change to the non-free VirtualBox (so I could use USB) but moving from one to the other in Ubuntu was not a simple process and eventually I rolled back (thank Christ for CloneZilla).
I’ve got a Win XP virtual machine with a 10GB hard disk which was getting full so I needed to expand it. The simplest way I found was to use the excellent CloneVDI tool. The tool runs perfectly under wine so I was able to double the size of the disk.
This did not however increase the size of the C drive in Win XP so I installed Partition Magic and resized C to take all available space. Now I’ve got a Win XP virtual machine with tons and tons of space 🙂