First up, let's get rid of that watermark. All you need to do is download and run a utility called Remove Watermark. You can find the download links at My Digital Life. Once you download the file, just unzip it, and right click the executable program and select "run as administrator." Make sure you choose the 32 bit version if you're using the 32 bit edition of Windows 7 and the 64 bit version for, well, the 64 bit version.
as the watermark hark, people will eventually make anti-expire cracks.First of, you can't get rid of that, so either get used to it or install another OS. Second, every Windows 7 RC is going to expire next July or so when Windows 7 hits the shelves for people to buy.
Edit, I stand correct by Skyriders edit. You can get rid of the watermark, but it's still going to expire.