Czuch: Save to disk (CD or DVD or memory stick) all data you want. It's best you save it yourself.

They are cheap enough (especially CD's) and you can make multiple backups if you want.

Then re-install if you wish.
But use a registry cleaner and a window washer after, as there will be alot of rubbish left over from all the bits you install. It'll keep your machine running faster.

But this is if your 'problem' cannot be corrected only.. as it takes hours especially if Microsoft are slow with all the updates you need to download.