For windows PC used in library, coffee shop, exhibition, even at home the once for all solution to keep it soil-free and virus-intact is to install “Steadystate” or “Deep Freeze”.
Deep freeze now has new version support for win 7. However, MS removed Guest Mode, the Steadystate for win7, from its formal release. And current Steadystate 2.5 only support xp3 and vista.
Other ways to deal with is to edit group policy with gpedit.msc or backup the optimized system with colonzilla so that it can be restored.