First, create a shortcut for a screensaver as told here
. Go then to Properties of your new shortcut and set a keyboard shortcut to launch it.
Notice that the post told above tells how to create a shortcut for "blank screen" screensaver but you can use the same method to create a shortcut for any screensaver of your choice.
To see names of the Windows 7 native screensavers open Explorer to be used when creating a shorcut, browse to Windows\System32 and type ext:scr
to search box.
If you have some third party screensavers installed and want to use one of those, search for .scr files in respective program folder.