This is the key that Windows asks for when you install it. It should be on a sticker attached to your computer, but if you can't find it, you can download and run the Belarc advisor which will tell you the install keys for all your MS products.
Re performance I would run MSCONFIG and see what's being started when you boot up. Usually you can find a whole bunch of things being started that you don't need and just take up memory and slow things down. Press the Windows key and press R. Type in MSCONFIG and hit enter. Click the Startup tab. You can uncheck anything there except your antivirus program although I wouldn't do more that 2 or 3 at a time. I would probably leave the hardware related things because they take the least resource and can come in handy.