Being redirected to another site!

I’ve had a few problems with my laptop in the last couple of weeks; whatever browser I used – Internet explorer, google chrome, firefox – the internet was running really slowly. Added to that, I found that when I first signed onto the JC boards, I’d click on a post and find myself being redirected to another site. This only happens with the first post, not any subsequent posts. Nothing obviously dodgy, sometimes it goes to Excite using the thread name as a search string, other times I’d go to random websites – this mornings was for custom made kitchens!
Also Internet Explorer started giving certificate errors for content on Facebook, and no one else I know seems to get this.

My virus checking software is running and is up to date however I did a system scan and it picked up 4 trojans which were dealt with.
Problem still there so I downloaded Lavasoft Adware…2.5 hours of scanning later, 1 possible fraud virus and the same 4 trojans that I thought had been dealt with.
Problem still there so I ran C Cleaner. this seemed to speed up the internet dramatically and the problem seemed to have gone away.

Until this morning, when the first thread I clicked on took me to the custom made kitchen website! Now some might say that was an improvement on JC (the kitchens were rather lovely but way outside my price range!) but I’m stuck for what to do next.

Any techies out there who can advise?
Thanks!