Regarding totalitarian regimes of the 1900s and unregulated Enron, WorldCom, etc, to what extent is it beneficial to regulate markets and...

Clearly, there is some benefit to having the government
regulate the economy.  However, too much regulation is just as dangerous as not enough
regulation.

In your question, you mention totalitarian
regimes, but those are not really representative of what happens when there is too much
government regulation.  A better example these days is a place like Greece or Spain
where excessive government regulation has helped cause slow economic growth and (now)
fiscal disaster.  In Greece, for example, there are laws specifying how much profit
pharmacists must be allowed to make. There are laws
designating how many people can enter certain lines of business.  These kinds of laws
drive prices up for consumers and stifle the kinds of competition that can make for
better products at lower prices.

Of course, laissez-faire
capitalism is not without its faults.  You are right to mention Enron and World Com and
you could also mention the problems that led to the financial crash of late 2008. 
Completely laissez-faire economies would suffer from all sorts of problems -- things
that existed in the early 1900s in the US like child labor and horrible working
conditions.  They would also suffer from things such as air and water pollution as
companies dumped wastes into rivers and allowed unlimited emissions into the
air.

So governments must always balance.  They need to
allow markets to be fairly free so that there can be innovation and economic growth. 
But regulation can be somewhat beneficial because it can prevent the worst abuses of
workers, the environment, etc. that come with a laissez-faire
system.