Search your PC for *.scr files. Typically, they are found in the C:\Window or C:\Windows\System32 folders.
Create a shortcut to your screensaver of choice. It does not matter where this shortcut is placed.
Right-click the shortcut and select Properties. Select the Shorcut Tab.
You have the option to enter a Shortcut key, in the format of Ctrl + Alt + <letter or number>.
This method is cumbersome, though, as it takes a short while for the screen saver to kick in. At least this is my experience on my PC.
What works better for me is to create a short batch file and place it (or a shortcut pointing to it) in the quicklaunch toolbar. One-click screen saver button...
Batch file as follows:
C:
CD C:\WINDOWS\system32
ssstars.scr /s