The first thing to do is push ALT and F10 on the keyboard to get Command Prompt up.
Change directory to C:\Windows and the launch explorer to get the GUI up.
Browse to C:\Program Files\Windows Small Business Server\Bin and launch the SBSSetup.exe.
This will then bring up the SBS Installation Wizard, I have found that to complete an installation of SBS when the wizard does not load on the first instance requires you to do this after every reboot. From memory SBS reboots about 5 times during the installation.