Computer companies typically package their computers with trial versions of AntiVirus software. Looks like yours expired. You can renew your subscription, but I wouldn't spend any money if it was me.

 There are quite a few very good FREE AniVirus programs out there.

 Top of the list is probably Microsoft Security essentials . I'd go with this one.

 Some others are AVG, Avira, Malwarebytes,etc.