The GUI insatller (setup.exe) for the Virtual Server 2005 Virtual Machine Additions do not offer a reinstall or repair option. That means that if you ever experience any problems with any of the additions you are forced to first remove the additions, reboot, reinstall them and the reboot again. But if you use the MSI directly with msiexec.exe instead of setup.exe you have some more options. This command will reinstall the additions on your virtual machine:
Msiexec.exe /faums VirtualMachineAdditions.msi
You need to mount the Additions.iso file and change to the Windows directory in the ISO image.
More info about msiexec.exe parameters here: http://technet.microsoft.com/en-us/library/cc759262.aspx