Thanks for your continued help!

I use the K-lite codec pack, an all in one solution which includes
versatile players such as media player classic.

OK, I installed K-Lite. It seems very good, although sadly has not
solved my video problems.

If you are installing/uninstalling programs as often as you state,
system restore points are critical to being able to keep known good
positions, to prevents situations just like this!

True, although easy to say in hindsight. I *never* usually have PC
problems, certainly ones I can't fix myself. This is a first for me.
I'd rather not have to reinstall Windows to sort this, so any ideas as
to how to get it working would be much appreciated.

Run DirectX diagnostics, see if directdraw, directshow and other basic
interfaces are functioning.

Yep, I ran dxdiag (or whatever it's called) and no errors were
reported. All tests passed too.

Back to square one! Any ideas as to how to proceed?!

Thanks very much!

Steve.