First of all, if you are using a pirated version of the game, leave now.
That being said, there's no need for you to download any patches. One of the reasons why steam is so popular is the fact that it handles all the patching and similar actions. You just hit the "play" button and you don't need to worry about anything else.
Files do get corrupt sometimes. If that happens, or you believe that it happened, open your steam game library, right-click on the game and pick properties. In the new window that will appear go to local files tab and hit the "verify file integrity button". Steam will work for some time and fix the game.
If it doesn't help, and when you change your hardware or significantly modify your system this may not work, just reinstall the game. Preferably directly through steam, not from dvd(s). It's faster this way and I use my DVDs only for serial numbers.
If your problem concerns mods, there may be a myriad of issues connected with them. I don't believe you're asking about that here but if you do, please go to the modding sub-forum where you'll find people more competent about such issues.