Ishbel - that used to happen to me too - oddly enough you can still use it - you just can't see what you are typing. Even with a pop-up blocker that ad still shows. Do you by chance use Firefox? Until I installed Adblock as an extension on Firefox it did get in the way (or whatever ad was there at the time). Now the ad doesn't even show.
Just go to another page to use that search feature.
"Count yourself...you ain't so many" - quote from Buck's Daddy
Errrm, my daughter (the tekkie one) installed all the software on my puter... I didn't think I used firefox, I THOUGHT I used IE... She's in Lebanon at the mo - I'll ask her when she gets back to the UK next Wednesday!