Tech-ish question

[vvvexation]

Something screwy has started happening lately: whenever I get an error trying to load a page, instead of the standard IE "page not found" message, I get routed to a page on www.internet-optimizer.com, to all appearances a fairly run-of-the-mill web portal. This is very aggravating because it means I can't hit Reload to see the page I was trying to view; I have to manually extract the URL from amidst the junk that it's been surrounded by in the address bar, or track down the link I got there from. Does anyone know how this sort of thing comes about (I know I've seen it happen before) and how to fix it?