Hi isolatedvirus
First of all thank you for the very detailed response. Here is the result, step-by-step
1) - 4)
Uninstalled Avast using their own uninstall utility. Tried pinging and streaming after re-boot but still no luck. I had actually rolled back the PC to a time before Avast had been installed, which didn't work, but I thought this was worth a try.
5) ipconfig/renew didn't work either. I had previously tried re-setting my router and then logged in to it to check there weren't any conflicts (which there weren't). This is also how I knew my router can see all my PC's and other devices and that they all had different IP's
6) I would never have thought of this and gave it a try to much anticipation. Alas, no good... still nothing
7) I have checked the sharing settings on everything so many times, I could do it in my sleep. Had one last run through though, but still nothing
8) Re-installed Avast which, to no surprise, as still nothing was working even when it wasn't installed, made no difference
Thank you so much for your help but I think I have to concede defeat. You've been very helpful but my I.T. training simply won't allow me to let you remote in to my PC. I'm sure you understand.
As this PC is now over two years old, the plan is to go and get a newer, faster laptop and bring that up to a level of install similar to this one and then flatten this one to factory settings.
The problem with this, as anyone who's done it knows, re-formatting a PC hard drive is simple enough and I do have all my install passwords and files backed up (twice)... It's the waiting for endless hours of Windows updates to install themselves when it realises it's so out of date. Hurrah for the Windows registry for breaking my PC and damn those updates!
Thanks again for all your help