Reinstalling Windows every once in a while?

So I've heard that people clone their windows image and programs to a dvd so they can fresh install every few months. Why is this exactly? Does Windows build up error and other files over time that make it slow down?

Every install of a program leaves settings in your system registry. Every boot of your computer has to read your system registry. Installing and uninstalling multiple programs/multiple times just adds to the overall size of the registry, increasing the boot time, increasing the use of the hard drive, increasing the need to defrag the system.

Even if you defrag your system every so often, your computer will probably slow down over time.

Each time someone formats/reinstalls windows, it puts the hard drive and overall system back to basic settings and faster speed.

At least that's how I've always understood it.

If you install and remove programs every other day, then you're going to start getting a bloated registry over time. I did a fresh install of my system last November but only because I made of hash of installing XP the first time when I built it (Jan 07). It left me with a partition under 200GB, on a 320GB HDD, so I NEEDED to do it. After 20 months, my system still ran fast as it did from day 1 with no problems or error messages, as if it truly had been installed from scratch.

If you use your PC conservatively, I doubt you need to reinstall. I think every few months is ridiculous unless you do system benchmarks or something and NEED a perfectly streamlined OS... but that's just being obssessive compulsive if the average joe does that.