Edited to remove superfluous and tedious detail.
I think I have been hijacked. My computer has been behaving strangely recently. Firefox, normally a superb bit of kit, has become unstable and crashes randomly. Today, I realised I had a full browser hijack on my plate. Google wouldn't let me search, as it detected my computer was sending automated messages, and every attempt to go to an anti-virus site resulted in a visit to a gambling den or travel agent.
I have wasted the entire day downloading various bits of software, sometimes using Anna's computer and a memory stick to get round the redirects, and then scouring the hard drive with electronic Brillo to get rid of the unpleasant stains. I think I have succeeded: Internet Explorer hasn't flashed up a window unbidden for hours now, and Google is Googling merrily away.
I'm still not sure what happened, but it looks as though downloading and running an anti-virus program called Avast has done the trick. AdAware, Spybot S&D, ZoneAlarm and CWShredder didn't find anything, but as soon as I ran a full scan with Avast (and it removed three files only) the abnormal behaviour stopped. And (touch wood) it has not restarted.
Update: I wrote the above and then went to eat my dinner, as we Northerners put it. I come back to the lapdog, and Google won't let me search any more, and my queries are being redirected to strange places. I wanted to print off my car insurance certificate and clicked on the Swiftcover link in Google (this was before it threw me out) and I ended up at somewhere called 'Getaquote.com'. Typing in the URL directly did the trick, so it seems only to affect Google clickthroughs.
Back to Avast for another thorough Brillo-pad-and-Domestos session, I think. See you later.
(Coda: Strange. I'm not a prude, but I am quite relaxed about all this as the sites I was being led to were all games, gambling, travel or other bland advertising. If they had been porn pages, I would have felt very differently, I think.)