i suggest using either the avg free or simply buying norton antivirus. i've used trials before.. and when your done if you don't want to keep using it, they mess with you. AVG's 30day trial messed with me REAL bad. I do a lot of benchmarking with my computer, it cluttered things up so badly that i had to actually reinstall windows. although i was happy with it. it found far more viruses than mcafee did previously.
my best advice: stay away from trials, either use norton or avg free.