FANTASTIC!!! Bloody brilliant!!! Thank you,Thank you,Thank you.

It's CLEAN,

I did a full scan with Malwarebyte and it found and removed infected files as I said, but left me with the big square screen. So I tried another quick scan and it came up with "no malicious content found" but I STILL had that bloody screen.

Then I tried system restore again and it let me in but after 6 attempts at different dates it wouldn't let me restore and left me with the big square screen.

I then tried Microsoft Security Essentials and it found 4 more infected files and cleaned them and hey presto, no more big square screen and it all appears back to normal so far.

I now have the Microsoft Essentials running 'cos "it worked for me".

My grateful thanks to all who contributed and wow, what a learning curve.

It's now nearly 3.00am but it was well worth it. Thanks again.