Are you sure you don't have some kind of malware, a keylogger or some such, running on your computer?
Is this the only site it does that with?
Are you using Internet Explorer as your browser? Whichever browser you are using, have you tried it with a different one? If so, did it still do it? If you are running Explorer and don't have another browser,
download Firefox - you'll be glad you did - then report back if this at least treats the symptom.
How old is your computer and/or keyboard (I have heard of keyboards getting 'viruses.')