Unable to scan...?

I just downloaded Avast today and ran my first scan. It did pick up 2 trojans and sent them to the chest. But all other files said unable to scan. Why is this? I’m not very experienced with theses things so please reply with that in mind. I have Avast protection set on high, I’ve had a lot of trouble with viruses and trojans in the past week. I also use Zone Alarm and AVG and run AdAware a couple of times a week. Is there conflict with any of these programs? I’m running XP.
Also, my download said it’s a 60 day trial… I thought it was a free program. Did I miss something or download the wrong version?
Thanks for any help.

More info about “Unable to scan”

http://forum.avast.com/index.php?topic=25030.msg205011#msg205011

avast! Home Edition is free to use, all you have to do is register it:

http://www.avast.com/i_kat_207.php

avast! 4 Home Edition works for 60 days after the installation in demo mode. You will obtain your FREE license key by E-mail after the registration. The license key is valid for 1 year. After one year you will need to just re-register.

I think avast! Professional have 60 days trial.

Thank you for your reply…very helpful on both counts.