If computer infected by the virus like sality, virut, vitro, etc, i can install avast 4.8 and do the mannual scan or boot time scan, and avast almost can remove the virus and all infected file. Despite the consequences, sometimes the windows system also removed and windows were damaged. But, if i try to solve the infected system using avast 5, avast 5 can to install but the service (real time) even avast gui can't running. I think it some of weakness. Three, when i do a scan over the network, often avast not detect any virus on the remote/other computer. But avast 4.8 can detect it.
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prevention is better than cure

there are some virus that is metamorphic, it can be identify only if infect your system. in avast 5 this virus do not have a change to infect the system because of file system shield this feature maintain the stability of every file that your system is executed. and do you think that virus definition of 4.8 is not present in 5.0??
will that is your observation

the only fault you did is, you did removed 5.0 and yes metamorphic virus take an action because file system shield is removed infect your system hide it in your system driver then you installed 4.8 and remove infected files and system files. your system become unstable and now you told us that 5.0 is weak.
man think that 5.0 offer you the best protection why do downgrade??? maybe to get an alert that your system is infected??
i use avast from 4.5 until now 5.0 and all that version i experience and see the changes in every upgrade.
Regards!!!