Here's my crazy idea for the day. It would be nice to be able to "virtually" install an app, say on a remote computer somewhere on the Internet. You could then try out the app on that virtual computer to see if you like it or not. This way, it doesn't screw up your regular settings if you want to uninstall.
Furthermore, you could do cool things on this virtual install, like having features that try to detect spyware (thru network packet analysis or thru an installation of LavaSoft AdAware on the virtual computer) as well as viruses. It could also detect if the app has any deviant behavior, like re-assigning what applications are associated with file extensions. At the end, it could come up with a summary of any deviant behavior.
This service might also be useful for software testers to see what it's like to install on various OS types, like WinXP, Win2K, Win98, etc. Imagine if you could say something like "do a virtual install on a 386 with a SoundBlaster card" and see what happens.
I could imagine a variant of this that would actually run on your regular computer. It could take a snapshot of your OS, virtualize it, and then provide you with a sandbox in which it would be ok to screw around with the settings and install things and even run potentially unsafe software, which would all be safely undone when you unvirtualize it. Sort of a variant of that virtualization software company bought by EMC (can't remember the name, the one that lets you run Windows, Mac OS, and Unix simultaneously... I don't know enough about it to tell if it can do this already).