Very few people I know start their IT career in software testing.
To be useful in software testing, you have to have some exposure to the software development lifecycle (SDLC). Software testing in an organized method requires a disciplined method of testing.
Its also good to know how people develop and code, because its those people that the tester will interact with.
Software test plans are based on software requirements - so knowing how requirements are written is also important.
So I would tell you that it would not be easy to take a course and land a testing job with no other IT exposure other than Office. Most of the testers in my company worked in support, became experts in the application, and moved to the QA team. Some have taken courses to expand their knowledge some have not. All of them were involved in some aspects of new release projects before they became testers. Some were coders.
STI has good information on their site on various aspects of the job
http://www.softwaretestinginstitute.com/
I've taken software testing courses from the Software Quality Engineering group (www.sqe.com) and directly from Rex Black http://www.rexblackconsulting.com/
I highly recommend Rex's books - not a bad way to find out more about the field.
James