My puter has a bug and I cant get rid of it!

[Knipples]

HELP!
I am pretty good when it comes to sorting things out on my PC, when you liv on your own you have to learn these things.
I have AVG version 8.0 and windows defender, both of which claim to be running normally and have done their daily scans, BUT the PC is very slow and I know it is sick. It is nearly 5 years old and that is part of the slowness, but I just know theres something up with it, plus for the last 3 days I keep getting this pop up link to some spybot thing (which just opens in a new window) that claims to remove harmful stuff on my PC. I havent clicked on anything on this page, and when it appears I just close the window.

What can I do to get my poorly PC better again, up until the last week or so everything has been running fine, and I have had no problems with the AVG or with the Windows defender. I'd really rather not go changing either of these because they have worked for so long but something is clearly on here which they arent finding and removing.

The AVG has found the odd thing in the last fortnight or so, trojans and stuff but its caught them and I have removed them.

Any advice thankyou please?