Now that I have Vegas Movie Studio 4.0 on the HD, can I uninstall Screenblast and Sonic Movie Factory (yeah, still have MF hanging around)? Or will this cause Vegas Movie Studio to not work?
If you installed Movie Studio in a new Directory (which you should have done), then uninstalling SB and/or VF will have no effect on the operation of Studio. They are completely seperate programs.
And if you did install them in the same directory, or if some unforseen problem causes Movie Studio 4 to stop working after uninstalling the others, simply install it again.