A fresh copy of windows have it's advantages: clean, fast, but are you sure you want to lose all your installed programs, documents and all updates ?
It's true that if you have windows skills you can save all your programs, documents when reinstalling operating system, but this require a lot of time.
It is faster to keep your windows clean.

First of all, we need to find out which are the main reasons why we need to reinstall windows: viruses, worms and slow performance.
If your computer is connected to internet, it is exposed to attacks, worms which trying to do bad things on your computer trough windows vulnerabilities.

You need an antivirus and a firewall, the best you can do is to choose a software that can be antivirus and firewall at the same time. I recommend Kaspersky Internet Security. In this way you don't have to worry about viruses, worms and trojans.

Now, the slow performance issue. What cause your windows to start and run slow: program installed which run on start-up. Some programs don't need to run on start-up, but such programs start some routine processes on windows start-up but you don't need this to happen. You will need a software to help you to clean windows start-up, i recommend System Mechanic, it has other tools which will help you to keep your system clean like removing unneeded entries from registry, remove junk and obsolete files. In windows start-up should be only those processes that are required for windows to work.

We are done with cleaning, but, what we can do if our windows stops working, or we do something that will cause damage to it. Well, if you go to Accessories -> System tools -> System Restore, you can turn on System restore. Whenever you install a new software, a driver or you make a change to your operating system, windows will make a backup, and if you don't like something that you made on your computer, you can just go to system restore, and restore your windows to an earlier date.

Last time i installed windows on my personal computer was 12 August 2004, and i don't have any problems. It is better than a fresh installed windows, because it have all updates, drivers and it is configured for my needs.