1: Assuming you just use whatever antivirus came with the computer (or came from your ISP), check if the subscription has expired, and either renew it or replace it with a free antivirus. Whatever you do, keep your antivirus updated and check it regularly.
2: Install, update, and run at least one anti-malware program, in addition to your antivirus.
Download links:
http://www.pcmag.com/article2/0,2817,2388652,00.asp