Not sure what could have gone wrong, but clearly the removal of Avast has damaged something or your system is infected, the best option is to use System Restore, let me know if this works:
Follow option 1 in this guide, if 'Repair Your Computer' is not available follow Option 2 and click on System Restore when you get to the Recovery Options, take it back to the restore point before you removed Avast.
Windows 7 System Recovery Options
Please tell me why you wanted to change to another Anti Virus, was there a problem with Avast?
Follow option 1 in this guide, if 'Repair Your Computer' is not available follow Option 2 and click on System Restore when you get to the Recovery Options, take it back to the restore point before you removed Avast.
Windows 7 System Recovery Options
Please tell me why you wanted to change to another Anti Virus, was there a problem with Avast?