IE - Pop-Up/ Redirect Problem

Hi all - Recently I've surfed to some legitimate but apparently un-maintained websites, including real estate pages, that have been co-opted to redirect to porn sites.  Now, on occasion, when logging on (dial-up) or in the middle of IE use I suddenly get about 4 pop-up windows going to these sites, some of which won't close.  The only way out is C-A-D'ing out and disconnecting multiple times, as they try to start a new connection.  This only happens occasionally, not every time I log on.  I've looked everywhere I know of, even dumping all my cookies.  There isn't a virus involved I know of as I scanned multiple times.  Is this what spy-ware is?  Any ideas? Thanks.