If your registry IS totally smashed then there's a quicker way of fixing it than reinstalling all of Windows. In a sense you can redo just the registry and leave everything else as is.
This would, however, mean that you need to reinstall every other program on your PC (a list of what's installed and the settings to go with each program is often in the registry) so really, it amounts to the same thing. The only difference is that you get to the "reinstall all my programs again" stage much much much more quickly.
Whether you do this (and I'm not yet convinced that something this drastic is necessary) or reinstall Windows, either way you'll need to reinstall every program again. Probably every last one, without fail.
A much quicker solution is to use commercial software in place of scandisk or defrag. e.g. in place of Windows' degrag, I use one called "Diskeeper"
Good luck!