Expanded disk allocation not recognised in Vista OS

I've recently installed Parallels on a MacBookPro - with Vista on the virtual machine. The problem I have is that I underestimated how much disk space to allocate to Parallels/Vista. I ran the Parallels image tool to add 20GB more disk space, but when I open vista it doesn't seem to have picked this up.

Is this a bug, or have I not followed the steps carefully enough to have vista pick this capacity up?

Help would be very much appreciated.

Thanks.//