I currently have AVG Free 8. Can I install Avira Antivir Personal Edition Classic 8 with AVG's real-time shield turned off and make it an on-demand scanner? Or can I turn off Antivir's real-time shield? Thanks..

Its not recommended even with real time disabled on either.
Although this may help to reduce the chance of your system locking up due to both software products attempting to access the same file at the same time, the real disadvantage may lie in two incompatible products causing system performance problems and a serious system slowdown.

You should only ever need one good AV as in this scenerio, two heads are generally not better than one.

Having said that, there are a few new 'multi-engine' virus scanners now appearing on the market which may be a better option in terms of avoiding program conflict.


Which one would you recommend then when it comes to detection and removal rates? Avira Antivir PE Classic 8 or AVG Free 8?

These are both very reputable AV's with strong variant database's, good detection rates and regular updating which will offer you par like, strong protection.
These days it gets down to very much a personal choice so I would recommend that you test a few out for yourself and see which best suits your needs.

All the latest freeware comparatives are available here: http://www.av-comparatives.org/

Personally, i'm a huge fan of Avast and im currently running v4.8 pro, but im always testing new versions and developments from alternative makes as they arrive!

Good Luck!