Restarting Computers

What is the purpose behind restarting your computer after an install or something, and why is it necessary some times and unnecessary others?

thanks

Most of the time you always had to restart the computer because the installation would add or chance variables in your registry. Up until WindowsXP the operating system would only notice the changes during OS startup, thus requiring you to restart the whole computer. Now other programs like antivirus or firewall need to load into a certain layer of memory along with the OS, which has to again be done during OS load. I'm sure there are other cases, but I wouldn't really know anything about them.

yeah, sometimes a program needs to be taken completely out of memory, or put something in memory, or sometimes there are temporary files that it needs to get rid of...