Check the add remove programs and see if there are any entries for other security applications.
With the ipod out are you able to boot normally ?
If not you could try uninstalling avast from safe mode and see if that allows a reboot.
Possibly a bit redundant now:
Sorry without any information on what that previous AV might have been it is hard to say.
You could try this tool and see if it lists any other security application - REVO Uninstaller freeware
http://www.snapfiles.com/reviews/revo-uninstaller/revouninstaller.htmlYou could check out the various download sites, snapfiles.com, download.com, majorgeeks.com, etc. and see if they have any registry cleaners that might find redundant entries related to security software. However, using registry cleaners aren't things to take lightly as they can ruin your whole day.