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For example you connect your laptop to your desktop PC and now can use the laptop as second monitor or even work on the laptop through the desktop computer. Or the other way round.
Note a lot of installers will legitimately load a one-off cleanup program at the next reboot, so allow them. It's great.
Another vote for firefox web developer tool bar. I honestly don't know how I got by before it. And a vote for Maxvista as well.