Hello I would like to know if there is some free antivirus to install on Asterisk 1.8?
Thanks
Asterisk 1.8 is not an operating system.
Or is it? 
Don’t confuse him more than he is!
Asterisk runs on many Posix-like operating systems, in particular several distributions of Linux. Any anti-virus would have to be running on that OS, not on Asterisk. Old versions even ran on Windows.
Generally the main purpose of anti-virus programs on such systems is to protect associated Windows systems, but a properly installed Asterisk system shouldn’t be sharing files with Windows systems, except, maybe, for call logging files, which are not executable. It shouldn’t be sharing risky files with external systems either. The virus checker is very unlikely to do anything to protect against toll fraud.
Virus checkers can compromise real time operation, particularly when updating definitions, so, if allowed to run during the day are likely to compromise audio quality.
Maintaining lists of virus signatures is too big a job to be done as a loss leader for something else, or based on voluntary donations, so any free virus checker will generally either be restricted to domestic use only, or will have its updating capability disabled. A domestic Asterisk system is particularly unlikely to benefit from having a virus checker run on its OS.