Indeed, I wish it was possible to try Windows from a CD before installing it like many versions of Linux Operating Systems like Knoppix, Ubuntu, and Mint.
I think MS has free trials, but they keep it hidden from the public, and installing them can be a bit of an hassle. I think if you used a regular Windows 7 disk to install onto another PC you have 30 days before your operating system gets disabled and nag you for a serial key which you have to buy.