You could also just install Windows 2008 on the 20GB and extend hte partition from the Disk manger afterwards.
It would save on time as well.
How to extend the boot partition in Windows Server 2008
1.Click Start, and then click Server Manager. In the navigation pane, expand Storage, and then click Disk Management. In the details pane, right-click the volume that you want, and then click Extend Volume. Follow the instructions in the Extend Volume Wizard to extend the boot partition.
Cheers,
Hades666