Cant sign into twitter?

when i go to a href="http://www.twitter.com" rel="nofollow"http://www.twitter.com/a in my browser it prompts me as if it is a download and ask if i want to save ot or find a prgram online to open it. im weary bc idk if this is some kind of virus or scheme. I never had to do this b4 and i was just on it an hr ago. HELP!