How to get rid of really sticky spyware/adware?

I'm pretty sure I have something nasty on my computer, it tends to freeze for short periods randomly and recently I started noticing adsense ads in places where they shouldn't be! When I check the source code of the page I don't see the ad there, but it shows up on the page.
I'm seeing it on both firefox and chrome, so I doubt it has to do with any plugins.
Tried scanning with spybot and malwarebytes and avast, but they didn't find anything.
Any ideas?

See original post by DomainMagnate