Is there an alternative to the 'insert' key?

Remove num-lock and press 0     on the numpad.

I just read somewhere that, for example on the Logitech LX-501 keyboard, the Insert is on one of the round blue buttons (next to the function keys). On some Microsoft keyboards one needs to hold down some Fn-key and press PrtScr.

So: are you sure there's no Insert key on your keyboard?

trex279's answer is probably the best one you're going to find but I thought I'd throw out an alternative:

Open the On-Screen Keyboard (on WinXP: Start > All Programs > Accessories > Accesibility > On-Screen Keyboard) and click the ins key.

Press Shift + 0 on the number pad to go into Insert mode.

On a laptop you may have to use Fn to get to 0 on the num pad.

E.g. on my Dell Latitude I had to press Shift + Fn + M to turn off overtype mode.

My answer is very specific to one particular situation. It will not necessarily work in other situations. I have a Logitech k750-mac solar wireless keyboard. I am running on a MacBook Pro accessing a Windows 10 desktop remotely. In this situation, I use the Shift + number-pad-0 key. But it only works with some software! Furthermore, I suspect the answer to this question will be more or less a table of situations with solutions specific to those situations, rather than a general solution.