Thank you to everyone who commented and looked to see whether I'd got pop ups or not! I went on the Blogger Help Forum and got a very helpful reply (see comments from last post) from someone pointing out that some website counters come with pop up ads (usually buried away in the terms & conditions). I've deleted both webstats4u and easy-hit-counters and that seems to have done the trick - but please let me know if you still get pop ups.
There's a helpful article here about the dangers of some free hit counters, as other people might be in the same position as me. I've also checked my computer with Spybot Search & Destroy and Ad-aware, as well as doing the usual anti-virus scanning.
This has been one of those weeks. Not only all this blog stuff going on, but I had tonsillitis again (well, it's not really news now is it), problems with my radiators, water from the shower tray going through the ceiling into the light fitting below and we ran out of bananas (not a problem for me, but a certain Somebody was a little annoyed).