Actually, any copy of Windows 7 can be installed without a license key and it will work for 30 days. You can "refresh" it up to 3 times to get a total of 120 days out of it before it will quit working (it was 120 days with XP, I haven't actually tried it with Win7 and it might only be 90 days).
That's fine for a trial, but you will eventually have to buy a valid copy.