What antivirus is it? (McAfee, Norton, Kaspersky, etc). It probably doesn't work, trials expire. There is no reason to use a trial, either buy one or get one of the many free ones out there. I personally us Avast on my home machine.
Be extra careful where you download from and use only well-know programs. Best to download from the software company's site, unless they give you the option to download from another, such as CNET or Major Geeks. Some of the better free antiviruses are Avira Antivir Personal, Avast, AVG, and Microsoft Security Essentials. I configured Kingsoft Antivirus and Threatfire to run alongside my Avira without any problems. Good free browser guards include Trend Micro and Rapport Trusteer. I downloaded Rapport from my bank website, so it would be configured to that bank's standards. Using a browser in Private Mode is also better. I have Avant, IE9 and Mozilla Firefox that I run in private mode.