MalwareBytes Anti-Malware nicely complements just about ANY anti-virus program of your choice. The FREE version [which is a scanner only] should offer no problems/conflicts whatsoever. The PAID version [which also offers realtime protection] may need some “tweaking” to avoid conflicts/slowdowns (if any) — but details on how to make things work together may be found here: http://forums.malwarebytes.org/index.php?showtopic=10138
I am using the Avast (Free) + MBAM (Paid/PRO) combination quite happily.