As with all past Windows Versions restarting will clear up some common problems. Restart your PC at least once a week, especially if you use it a lot. Restarting a PC is a good way to clear out its memory and ensure that any errant processes and services that started running get shut down.
Restarting closes all the software running on your PC, not only the programs you see running on the taskbar, but also dozens of services that might have been started by various programs and never stopped. Restarting can fix mysterious performance problems when the exact cause is hard to pinpoint.
If you keep so many programs, e mail messages, and websites open that you think restarting is a hassle, that's probably a sign you should restart your PC. The more things you have open and the longer you keep them running, the greater the chances your PC will bog down and eventually run low on memory.