There are a number of mods for crossbows as mentioned, not sure how good they are though. If I justify it for my character's MO I might try one...
[EDIT] then again, following some investigation, perhaps not...
Hmm? The Crossbows for Cyrodiil Mod slotted beautifully into my Oblivion game. In most cases I prefer to use an ordinary bow anyway, but they were very well done and I was only playing a Beta version of the mod. It wont harm you to try it, and I really enjoyed it.
Shame there are no crossbows in the vanilla game. They're an awesome weapon and fit some characters perfectly. Unless you have the PC version of the game, standard bows are going to have to do.