Found this after clicking a link to Play's everything £5 deal I had on email this morning.
After seeing many posts on here for this it seems you can indeed use the key to update it to the latest version so to protect 3 PC's for a fiver I thought was a bargain!
Today, threats to PCs are appearing in ever-greater numbers and diversity. Cyber criminals on the Internet are well organized and tend to target PCs with combined threats. A standalone antivirus solution is no longer enough to fully protect your PC.
Kaspersky Internet Security 6.0 is a fully integrated solution that protects your computer from all of the most common Internet threats, including viruses, hacker attacks, adware, spam and spyware.