Hi andrewfree and welcome to the site.
From your post I am going to go with that you know your way around a computer however you don't have anything formal to back that up from the point of view of a prospective employer.
One course I would recommend is the CompTIA A+ course as a starter, as it teaches about computer architecture in particular hardware and some of the basics about networking topologies. It also takes you through various Microsoft Operating Systems such as Windows 9x right through to Windows XP. (It may now be updated to incorporate Vista, however when I was doing it, Vista hadn't been launched).
It is a very general course, and you may find you it is simply reaffirming knowledge you already know, but I would say it provides a fairly good grounding.
To be honest though in the real world, you can have as many exams on your CV/Resume as you like, however most employers value experience over qualifications, although they don't do your chances any harm. You'll see from adverts that many IT Engineers are sought after with a minimum of 2 years experience. If you are starting out, then try getting a weekend job if possible to get some experience....however you may be lucky enough to find an employer who is willing to take you on and train you up.
What area of IT do you want to specialise in?
If you are planning on working with mainly Microsoft systems, then perhaps look into Microsoft’s MCSE or MCSA qualifications, or even the MCDST. Have a read here:http://www.microsoft...se/Default.mspx
If you have any other questions, please do not hesitate to ask!