Is it acceptable if my boss open and use my computer when I'm not present at work?

noooooooooooooooooooooooooooooooooooo

Every Employee has to follow the Business Etiquette/Ethics/Company policy and this applies to the  Bosses as well.  So, without your Permission your Boss cannot open and use your Computer.

not acceptable at all without a premision from me first .

No,

This is not acceptable and it is invading your privacy.There should be policies in place preventing this kind on action.Unless you are the only one who has the information he requires and contacts you first, it should not be permited.

Thanks

No it is unacceptable, unless there is an important issue 

Its against the IT act (if any in the organization). If its so urgent he should take permission on call and if you are not available then he should ask IT department for access to certain files in your system as per policy(if any). 

If it's something work related and urgent - the boss must first take permission on phone from the user and then access anybody's PC. It is called 'Personal Computer' for this reason!