Do you observe the same behavior when typing slowly? I did the one- finger typing thing and the same thing happened.
Are you pressing any modifying keys with "Y"? Not that I am aware of
Can you type "Y" in notepad? I'll give this a go when I get home and let you know but it has happened when using MSN, Word, Powerpoint,
Do you run any software that adds hotkeys? no not that I am aware ... apart from MS Office, MAN Live Messenger and Comodo Security on.