During the Avast download process, it would not ask for ISP or attempt to change it.
Were you downloading anything else after Avast?
At this point, I can't tell you whether to worry or not because I am not sure of what you have done.
How does the system appear to run?
Do a full Avast scan to see if everything is "clean".
Right click on the A ball in the tray and then click on "Start Avast". When the GUI (User Interface) loads, click on the little icons to set the on demand scanner (Avast) to scan your hard drives. Also set the "slider" to Thorough with Archives.
Also, the next little icon is for external and removable drives. IF you have any, then set Avast to scan these too.
We will take it from there.
PS: You didn't have NORTON installed prior to Avast did you?