Wifi trouble.

I'm having trouble with my Wifi connection on my desktop. It connects to the network and says there's an excellent signal but frequently says there's no Internet, even if I'm sat connected to the Internet on my tablet. If it does say it's connected it's painfully slow to the point of uselessness, pages don't load properly or don't load at all. I can connect to Steam maybe half the time.
I'm using Windows 7 on an installation less than 6 months old on a solid state drive, and I'm using Avast antivirus. I've changed my router, changed my Wifi adapter which worked for 2 days then became even worse, run endless scans to no avail.
Anyone got any ideas?