I have the same music collection on two computers, on one of them I have updated the ratings, is there a way I can transfer these ratings to the other computer?

Try Syncotunes or this comment

Can't you just copy the library file?

Pardon my ignorance, how do I do that? And will it affect the songs in my other library?

Assuming both are on iTunes 9, try the "Home Sharing" feature, which is designed to sync two or more computers on the same network, and using the same iTunes Music Store login details. In the sidebar, click on Home Sharing and follow the prompts.