I've been looking for some off-the-shelf software that will allow me
to pop up 'tip' boxes on the screen when I, 'the wizard' make certain
keypresses etc.).

Do you know of any software that already exists that can help me with
this?

I'm looking for some simple software that is unintrusive (i.e. is
invisible until keypresses are made) and simply pops up the 'tip'
boxes in various pre-defined positions on the screen over the user's
browser when I, the wizard,  press certain keypresses. I'm looking for
pointers to ready-made software that can be used to achieve this (all
of the above functionality, not just part of it).

It might help others if you could clarify your request.

You say the 'tip' should pop up over the user's browser.  Are you
thinking of just one browser e.g., Internet Explorer or should the
software (if it exists) be able to generate tips for any browser?

Your mention of a browser makes me think you want your tips to appear
on a website.  If that is the case, why not code the page to generate
the tip?