With a program as vital and as important as Avast! is to a machine, patience or researching an issue on Google is often the best thing to do before any course of action is undertaken. Posting here in this forum is also a good idea; it helps to soothe one's nerves as the above is true.
As for the machine being stuck in avast setup, the best course of action, unfortunately, is to try an Avast! repair, as DavidR stated earlier. If that does not work, a clean install of the program is best. If using a paid version of the program, you will need to save your license first somewhere where you can find it later to resume full functional use of the features in the paid program.
(As you posted again, in between, keep this process around should you or anyone else ever need to use the steps described below: Glad your issues are now resolved.)
Attempt to repair first, if that does not work
, come back here for instructions to do a clean install. I am currently active and watching this topic to assist you should you need it.
EDIT: Your post came in whilst I was typing here, so..
You will need to use the other machine to download the following file here: http://www.avast.com/download-software
Select the version you are now using.
THEN: Go here and get aswclear.exe and download that here: http://www.avast.com/uninstall-utility
IMPORTANT: Be sure to save aswclear.exe only to the desktop and nowhere else when transferring this file to your problem computer. Read the page linked carefully to be sure you understand how to use this file, very important.
NEXT: Try to see if you can uninstall Avast using Add/Remove or Programs and Features. This may or may not work while avast! setup is still running. If this fails, next step is to go to Safe Mode and uninstall from there using either Windows uninstall program.
Reboot back into Safe Mode. Double-click aswclear.exe and be sure the browse button has you pointing only at the AWIL/Avast! folders, as is noted, it will remove all traces of files and folders it is pointed to.