Avira is not available because its developers are not capable of writing 64bit drivers. if you are choosing not to move to 64bit because of avira, maybe you should replace avira with something better. like MSE or McAfee or Norton.I'll give you an example,
Avira Antivirus has a HIPS component called "Avira Proactiv", which is not available in 64bit os(7/8.1) due to kernel patching restrictions
also the same case with Comodo Firewall,
effectively any user using any antivirus and firewall in windows 64bit are covered a little less than their 32bit counterparts.
its just one example, but there are many "compatibility" issues like these.
PS: why would MS let some third party patch its kernel? its a security threat.