As per the downloading of files again, if you right click on the steam icon in the taskbar, there should be an option to 'backup game files' in their somewhere, use that so you can put the files onto a CD, and reload them.
Once the game is reinstalled you shouldnt have to reenter your CD-key because you should be able to use the steam login that you made - this login should be directly related to the CD key of your game. There is probably an option in steam at the CD key enter screen or just before to login to your existing steam account.
Hope that helps.
Perhaps someone that has physically done a reinstall could confirm or deny aspects of what i just said.
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