Re: SketchUp Questions...
What kind of machine? A PC or a Mac? PCs have a pretty robust install-uninstall system, and it should be easy enough to go to the "Programs" section of the Control Panel and uninstall 8 before you re-install 7.
You might lose some preferences and custom menus, though. I'm not familiar with the software, but they usually keep these things in certain files, and if you copy them to a completely separate folder, you can put them back after you re-install 7.
Of course, I'm assuming your installation of 8 did not write these over and erase them.