It may be you can't install KAV because you haven't removed all of McAfee.
I went from McAfee Av to KAV 6 so it can be done.
Click "start" - run and type: msconfig - untick any applications that begin with "Mc"
Start up in Safe Mode (keep pressing F8 during the boot process) - go into add-remove program and remove every McAfee thing you can.
Then search for "McAfee" file and folder and delete them.
Restart, and you should be Mc free - you might want to check and remove the McAfee ActiveX component - best done using Spybot (advanced view - tools).
KAV is by far the best AV I've experienced and worth a bit of aggravation.
I don't know if the free trial has a firewall, if not try Zone Alarm (free) click here