>
>>Also, can you elaborate on what's weird about the 404 error?
>
>It said "404 error" in the titlebar, but the page was blank with an
>extra scrollbar in the page area.
>It's gone now, so maybe actually having someone else look for the
>problem fixed it :)

That sounds like something's trying to wrap the page in an invisible iframe.
I'd suggest both scanning your machine for malware, and making sure you're not behind a (hotel-style) web proxy that's trying to inject external content into your web browsing experience.