Having recently ditched the under-performing Microsoft Security Essentials for something better (in my case, the free edition of Avira), I know the pain of trying out new software. Anti-virus programs can sometimes be stubborn about removing themselves and if you're finding the ghosts of old AV apps lingering on your system, ESET has an all-in-one removal tool that can help.
It's a free download from ESET's website and comes in flavours for Windows 7, 8, Vista and XP. 32 and 64-bit executables are also available, so the popular Microsoft operating systems are all catered for.
Strangely, on my system the program detected Malwarebytes Anti-Malware, but Avira was missed. Looking at the list of supported apps for Windows 7, Avira does appear, but only up to version 13 — I'm running 15. So newer AVs haven't been incorporated yet, which is disappointing.
On the bright side, ESET also provides links to manual removal utilities for a number of anti-virus applications, so you'll hopefully be able to find the right tool for the job.