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To do that, you'll need to do a little bit of work and searching...
Microsoft attempt to prevent people from using themes in Windows that aren't made by them directly, so you'll need to use a workaround. UXStyle is the safest option as it doesn't modify any system files, but rather runs as a service that disabled the theme signing check and therefore allows custom themes (link).
As for finding themes, there are many sites around that have galleries of themes to download from, but I'd suggest you look at DeviantArt first, there are some really awesome themes there and the users are very helpful when it comes to answering any questions (click here for Win7 themes).
One of our SevenForums regulars also has a lot to do with skinning and themeing Windows (and other apps); check out his DeviantArt profile page here: sergiogarcia9 on deviantART