i am using avast 4.8 home edition with the latest update, after the update bitdefender virus shield can’t be started, 
bitdefender also can’t get update, avast detects the file zlib.dll in bitdefender program as trojan. I am quite sure this is a false positive, is there anyway that i can disable the blocking? ??? I am using bitdefender V10 free edition, avast and bitdefender had been quite ok with each other until the update. Please help, thanks in advance.
It is a false positive, you can relax.
The newest update of avast! fixes this problem.
I went through the same trouble as you and the avast update I downloaded minutes ago, showed everything it previously listed as infected as clean again.
No more blocking of BitDefender either.
Just update the avast! database again 
Leong
are you trying to run both avast and bit defender real time load at startup style???
FYI
“after the update bitdefender virus shield can’t be started” ???
run avast installed real time and just use bit defender as on demand scanner
you DO NOT want to have TWO AV’s installed at the same time-
did your machine come with an AV preinstalled??
You can only have ONE resident antivirus running on a PC, two is prone to disaster! 
BitDefender free is on-demand only so shouldn’t be a problem, I would simply suggest you pause the standard shield whilst doing the BitDefender scan.
My post was to cover the possibility that OP had a PAID copy of Bit Defender
although the free scan installs lots of programs so it may look like a “virus shield”
my question was on the definition of “virus shield” which I would not expect in a free version
I saw this in the OPs original post.
Whilst I quoted gdiloren post and not your directly, that is why I made the comment, re BitDefender free and being on-demand.
Thankfully it isn’t a clash or conflict between AVs but one of a probable FP detection of a file.
missed it in OP first post