When working with certain systems one might have to install some antivirs software. The Personal Edition of Avira Antivir does its job pretty well, I think: at least I feel safer by having some magic virus catcher installed :-) However, every time Antivir gets its (daily) updates, a nag screen pops up. Here's how to disable this popup ad:

Start, Run, secpol.msc
Right click "Software Restriction Policies," choose "New Software Restriction Policies"
Right Click "Additional Rule" folder, click "New Path Rule"
Where it says Path, Type the path of avnotify.exe on your computer
Make sure the "Security Level" Dropdown menu is selected as "Disallowed"

This should be it. Thanks, Wikihow editors :-)

Update: there's a new nag screen, listing current offers and urges one to buy their premium version. I'm fine with the free version but I'd like to get rid of the advertisments too. Adding some more path rules to the Software Restriction Policies and disallow their execution above should do the trick:

ApnIc.dll
ApnStub.exe
ApnToolbarInstaller.exe
avnotify.dll
avnotify.exe
ipmgui.exe - ?