If you found out your company was doing something against the law, like fraud, what would you do?

Company can't do nothing against the law, and company can't do fraud at all. Everytime it is an specific person, or group, never the company. 

And any person within the company, any staff member is managed by somebody, have some boss. So talk with the boss, talk with the company lawyer, talk with the LPS head if exist or escalate your suspicion to the owner. Because - from one side - you must be loyal, but from the other site - you need to protect your company, its executives and owners. And their values and assets.

Good thing would be to escalate to the above or whistle blowing in order to protect your side.

Following the Rules and Regulations of the country which I am leaving is a part of my religious belief.  So, defenitely I will not support it.

To be a good county man i may not support it.

but when it comes in terms of professionalism my reaction will be "Its non of my bussiness what company is doing". I have nothing to do either company is doing right or its going against law. then it will coms to my self... if my inner allows i will do job or in other case i will diffiently put a resign note... :)   

People in different capacities would react differently, as "CAE" he/she must immediately report the matter to the audit committee and the board.

I will blow the whistle, every bank has whistle blowing policy as per that we will be providing information about the things that are going wrong in the company to independent body like HR or risk or internal audit as these functions are not directly reporting to Business or CEO

Mostly, this might happen in small companies or organisations, but if we are talking about big companies I think fraud will be an act of one employee or a group of employees which can be divided into three categories:

1- Fraud that affects customers.

2- Fraud that affects the company itself

3- Fraud that affects government

each category is different, and the reaction to me finding out that fraud should be different too.

But in all cases this discovery is not supposed to go public and have to be resolved either internally, escalated to internal audit/external audit, escalated to board/investors, or escalated to police.

I repeat, I don't think big companies are welling to risk their reputation to commit fraud.

I believe that it is rare and difficult that company as a whole itself is involved in a fraud. There might be different people or group of people involved in doing that fraud. In that case, being an internal auditor matter has to be investigated properly and then shall be discussed with the respective managers, senior management or the board. And if its a company as a whole then wise decision is to discuss this matter with the board first and if they are already aware of this and are not interested in fixing this fraud or are part of this, wise decision is to withdraw working with them