I'm guessing it's a 2004 Trek 5500 triple - the 5900 had an OCLV 110 frame. The '04 5500 double had a 10-speed Dura-Ace group, while the triple had a 9-speed Dura-Ace group.
edit: plus, that straight-bladed fork, according to
this handy little timeline would suggest that it's a bike from 2003 and onwards. Don't know if I'm correct, but it seems likely.