Should I be worried about new virus protection?

I was perusing the internet to kill time and a message appeared that said my laptop has a virus (it didn't specify what it was) and that it should be taken care of asap to prevent from losing any files. The laptop was given to me from my big bro, and I haven't really worried about protection til now. There's a McAfee Security Scan Plus software already installed, but the laptop cannot find any active virus protection. The Windows 7 software site has the BullGuard software free for 90 days, and I want to download that for now, checking to see if it will take care the problem, and then download the 2 year virus protection for $60. Do I need to download a new virus protection? Are there free alternatives that are just as effective and reliable?