Question Author
What an exceptionally helpful bunch you all are out there!
Thanks for each and every one of your responses, and for the actions you've suggested. I've not had time yet to work on any of them, other than running another full AVG scan - which doesn't seem to have made any difference. At the end, it said '0 threats detected', but the tags are still there.
I'm having so much time wasted by the problems with this computer that it's going to take me a while to get anything done. As I said, it's been getting slower and slower recently, and keeps crashing. It takes ages to reboot it, and switching it off and back on again is the only way to deal with it when it refuses to scroll or change page, and keeps giving me 'not responding' at almost every key press or mouse click.
I'm not a very computer literate chap - my only brief training was a short course run by my local library in 1999, which only taught me the basics of operation. The computer I have is an ex-internet café machine. For about 10 years I was a regular customer at my local shop, but unfortunately, in 9/13, they closed, and there was no nearby alternative. But the owner was grateful to me for my long-standing custom, and for my reliability in manning the front desk for short periods - so when they closed, they gave me one of their machines which, he told me, had been completely cleaned up of all the rubbish that had got into it from years of use by customers.
It worked reasonably well for about 6 months, but eventually started giving problems. I have no instruction manual, and the questions that some of you have asked about which system I have, I cannot answer clearly, and many of the terms involved mean little to me. I can only tell you that it is a very heavy machine, with no clearly identifiable branding other than Microsoft. The browser? Well, I think if I tell you I'm using Mozilla Firefox, that's probably the answer to that one. But what system? I have no idea! Sorry I can't be more helpful there - I was just given this machine, for which I was very grateful in my long-term benefits claimant status, and all I've done is use it the best way I know how. Maybe I've accessed a little too much porn... which I know can draw in all sorts of malware - but I use it mainly for YouTube, Amazon, Discogs, Google, Wikipedia and the websites of the BBC and the other main broadcasters.
I also have thousands of pictures and URL links saved to both my e-mail and desktop, which I'm told may be overloading the memory. I'm told that I should save all the stuff on my desktop to a USB stick - but nobody ever showed me how to do that!
So I'm not sure which piece of advice you've given me would be the best to follow. I'm concerned that I may get myself tangled up in a situation which I don't know how to get out of if I try, and possibly end up losing all sorts of saved information in so doing. A friend who lives near me has suggested that I go for a 'system restore', to reset the machine to a point before these 'ad tags' appeared. Any thoughts on that one?
I still have the number of one of the guys who ran the internet café, who told me to call him if I had any problems with the machine, so I'm going to call his number soon, and see what he has to offer. He was usually pretty good at getting all the overused (and abused) machines in the shop to behave themselves!
So thanks once again to all of you for your helpful suggestions - I'm sure that somewhere in there, there is a solution - it's just going to be a question of finding the one that works, while not getting some further problem that's going to wipe out all my files and saved stuff.
I'll post an update of how things go. All the best.