I have a software company in Canada and I am starting to sell software
to US customers. To do this, I have to go to the US to do initial
installation and I will most likely go to the US as required to do
upgrades and trouble-shooting work. Do I need a visa for these
purposes? If yes, what is it? If no, do I just tell the Immigration
Officer that I have to go to the US to do some work? They are known to
be very strict on 'work' related entry? Do I have to pay any tax on my
sale? on my maintenance work?

Immigration issues for after-sales services
    
Repair, warranty, maintenance and related services are described under
NAFTA as work done by:

        "...installers, repair and maintenance personnel and
supervisors, with specialized knowledge essential to a seller's
contractual obligation, performing services or training workers to
perform services, pursuant to a warranty or other service contract
which is incidental to the sale of commercial or industrial equipment
or machinery. This includes computer software, purchased from an
enterprise located outside the U.S."

    U.S. immigration officials can be quite sensitive about
after-sales service. Hands-on building and construction work do not
fall under the after-sales service provision of the NAFTA business
visitor category, so you'll need to prove that the work you'll be
doing inside the U.S. does, in fact, fall under the NAFTA regulations.
To do so, you'll need a copy of the original sales contract, which
must clearly state the service work to be done. Showing a company
letter that describes the reasons for the business trip is also a good
idea.