[ubuntu] Expanding Disc Space in Oracle Virtual Box (Win XP guest system)

My Win XP virtual machine is close to using all of the disc space it has been given. I have Googled what to do and have come away more confused than anything. I have tried this command in the terminal:

VBoxManage modifyhd./home/myname/VirtualBox VMs/WXP/WXP.vdi --resize 20000

The /home/myname/VirtualBox VMs/WXP/WXP.vdi is what I found in the VirtualBox program, maybe I am using the wrong terms here?

Also what is the best way to expand the partition in Win XP once I have given it more to use?

Thanks

Matt