|Windows Defender is superseded by Microsoft Security Essentials (MSSE).|
AVG PC Tune-up 2011 is a magic fixall program rather than an Anti-Virus so this should not clash with an Anti-Virus.
MalwareBytes is well worth having but you can simply run it when you wish (rather than as a background program). Keep it updated.
As already said, only one Anti-Virus or they will fight with each other. Using your list that is one of these:
Windows Defender (well, almost an A/V)
A lot of folk do not seem to favor McAfee these days.
Windows Defender is outdated and limited.
Webroot - Sorry, I don't know a lot about it.
Avast seems popular.
I would be inclined to uninstall your troublesome Windows Defender and install either Avast or MSSE instead. If you do this McAfee and Webroot would have to go too.
How to know you are getting old 5:
A lamp post backs into you