Thanks for the replies, guys.
I'm not using a firewall [although it's a 2nd hand PC - how would I check that there wasn't one I didn't know about which had somehow become activated?]. And no, I'm not getting the 'page cannot be displayed' thing - I'm familiar with that, and know that it's usually a fault with the website in question. The sites I can't access are fine; it's just my PC that can't access them. The page that comes up is usually just completely blank but with the bars still in place. It looks as one would expect to see the webpage look but with absolutely no content! The only anomolous occurrence I can think of recently is when I reconnected my PC and tried to get back online (bearing in mind I am not enormously IT literate). I wasn't having any success so I tried running the modem installation disk. In fact the modem wasn't the problem - the computer had automatically gone to the dial-up default setting (remember I have broadband) so I just had to cancel that out. Could having 'reinstalled' by modem without 'uninstalling' it first be the cause of my problem? [Hope all this is making vague sense]. Your help, again, much appreciated.