Both of the members above are more experienced than I , and are correct.
But be aware - you have to reinstall all of your programs. If you are missing the installation source, or the product key, you will have problems running the old software. You cannot just copy it from the old disk.
So if you have all your ducks in a row, it's a boring process that leaves you with a nice, squeaky-clean installation that runs faster than your old installation did.
Do backups every so often so that if something goes wrong you don't have to start from the very beginning.