Have you been fiddlin' with the HOSTS file?
If your HOSTS file contains an incorrect or outdated listing, you won't be able to connect. Even if you don't recall adding listings to a HOSTS file, it still might contain listings, because some Internet accelerator utilities edit them without telling you. Open your HOSTS file with Notepad and see if the site you can't connect to is listed there. If it is, delete the entry, and you should be able to connect.
Made any changes to your DNS settings?
Recheck your ISP's DNS settings.
The problem might be related to your DNS cache, so flush it out.
To flush the cache, type ipconfig /flushdns at a command prompt.
Mike-B, Green G, et al, should be along shortly to give a more focussed view - I never get it right
but learning all the time!