I am glad you were able to get this done in this manner, but you can (and I did) use the GParted tool for my Windows XP host by following the helpful entry by Eric. To re-iterate/expand on his solution (don't be afraid of the # steps, I'm trying to help newbies here, so there are necessarily more detailed instructions!):
change the size of the virtual hard disk via the VBoxManage modifyhd command, which is well-documented here and in the VirtualBox documentation.
download the GParted-live (http://sourceforge.net/projects/gparted/files/latest/download?source=dlp) or search the internet for GParted-live ISO. The important part is to get the live (.iso) verison, which is in the form of a bootable .ISO (CD) image.
Mount this new .ISO to the CD virtual drive in the host machine's Storage settings
If necessary/desired, change the boot order in the System settings for the host machine, to boot from CD before Hard Disk (alternatively, you can press F12 when it's booting up, and select the device)
start your VM; if you changed the boot order, it will boot to the GParted-live ISO; otherwise press F12 to do this.
do not be afraid or get too confused/wrapped up in the initial options you are presented; I selected all the defaults (booting to GParted default, default key mapping, language (assuming English - sorry for my non-English friends!), display, etc.). Read it, but just press enter at each prompt. With a Windows VM you should be fine with all the defaults, and if you're not, you're not going to break anything, and the instructions are pretty good about what to do if the defaults don't work.
it will boot to a GUI environment and start the GParted utility. Highlight the c: drive (assuming that's the drive you want to increase the size on) and select resize/move.
change to the new size you want in MB (they abbreviate MiB) - just add the new amount available (represented in the bottom number - MiB following) to the middle number. E.g: I changed mine from like 4000 MiB (e.g., 4GB - my initial size) to 15000 MiB (15 GB) because I'd added 10 GB to my virtual disk. Then click OK.
Click Apply. Once it's done you'll have to reboot - for whatever reason my mouse did not work on the desktop icons on the GUI (I could not click exit) so I just closed the VM window and selected reboot. I did not even have to unmount the ISO, it apparently did it automatically.
Let Windows go through the disk check - remember, you just changed the size outside of Windows, so it has no record of this. This will presumably allow it to update itself with the new info. Once it completes and you log in, you'll likely be told that Windows needs to reboot to use your 'new device' (at least in XP it did for me). Just reboot and you are done!