You'll see a dialogue box asking you why you're choosing to uninstall the latest update; tick all the boxes that apply and click Next.
Many times the updates are downloaded but are scheduled to get installed automatically during Windows shutdown or startup.
I have a fifth generation i Pod, and I recently bought a new computer already installed with Vista.
When I plug in my i Pod to update my music, it will appear to begin syncing but the go to the Blue Screen and restart my computer.
Uninstalling a recent build will affect changes made to your computer since that build has been installed.
This includes apps and programs you've installed, as well as any changes made to settings.
It should not affect your files -- even files created and saved after the build was installed -- but it's still a good idea to back everything up just in case.