All other things being equal, how often you should shut down your system depends on what sort of software you spend most of your time running.
All software, including Microsoft software, has bugs and will generate memory leaks under some circumstances or other. These leaks can build up and sap your system resources to the point where the computer bogs down completely. Then you get the "low system resources" error message, usually followed by a crash. ("system resources", by the way, doesn't refer to your computer system as a whole, it refers to a surprisingly small slice of RAM which Windows uses to track certain special objects).
If you spend much of your time running freeware and shareware, which don't get the same degree of professional, extensive debugging as the name-brand products, you should shut down once a day just to clear the memory (apart from all the other good reasons mentioned in the other posts!). If you spend most of your time running (eg) MS Office applications, which are relatively good memory managers, once a week or even once a month could be enough.
