>>>I keep getting asked for money even though I downloaded the FREE AVG
I suspect that either you've downloaded a trial of the full (paid for) AVG security suite or you've downloaded from a dodgy site.
However, after using AVG for years, I ditched it when I found that it had started slowing my computer's performance. I now use the 'Essential' (= free) edition of Avast
http://www.avast.com/en-gb/index
which I'm happy with.
In addition to an anti-virus program you also need a firewall. Many people are happy to use the one that's built into Windows XP (and enabled by default) but some people prefer to use a (free) alternative. Comodo has long been one of the best suppliers of such a firewall:
http://personalfirewall.comodo.com/
(Use the right-hand link).
However Comodo has more recently also become known for its free anti-virus software, which offers an alternative to MSE, AVG or Avast:
http://personalfirewall.comodo.com/
(Again, use the right-hand link).
Reviews of anti-virus programs in computer magazines never seem to agree upon which is the best product but 'Avast' seems to be the 'flavour of the month' with several of them at the moment (although I've also seen Comodo well-reviewed).
However you should note the following:
1. You should only ever have ONE anti-virus program on our computer. Two or more can cause software conflicts.
2. You should only ever have ONE firewall running on your computer, for the same reason.
3. You also need anti-malware software but you've already got the best anyway (Malwarebytes).
4. You can have as many 'run on demand' anti-malware programs as you like as they can't conflict with each other. So you can add further anti-malware programs if you like.
5. While Microsoft are withdrawing support for XP, per se, they are NOT withdrawing support for Microsoft Security Essentials which will CONTINUE to work with XP. (However MSE failed quite a few independent security tests last year, forcing Microsoft to admit that it's not as good as they'd want it to be, so it might still be best to ditch it anyway).