ahmad:
I wonder if installing Avast would have increased the boot time (to the point of the logon screen)? But I don't know what Avast would have anything to do with that.
Sometimes, the bootup (atleast from login to desktop) will be longer if you have installed some anti-virus/internet security apps and also depends on the startup programs. Avast may be doing some initializations or some initial scan once you login to your desktop. I am not sure. Best is to see what programs you still have in the startup by typing msconfig from your Run dialog box.