I used this on two bathroom floors with large grout lines and I can't say enought about it. It's really easy to use and makes the grout look brand new and lasts. You basically paint it on and wipe off the tiles.

Here is a link that might be useful: Grout refinisher

That's a grout colorant. If you want to go that route, I can give you the url to a premium one. You might try hitting the joints with oxyclean and a toothbrush, first, though. Even if it doesn't clean it good enough for you, you have to clean the joints ANYWAY before you use a grout colorant, so it won't be wasted time. Oxyclean won't make the grout look like new-- especially old food stained white grout-- but it'll make it look a whole HELLUVA lot better than it does now. Start with that, and if it doesn't do a good enough job for you, come on back, and not only will I give you the url to the product, but I'll also give you another url with step by step instructions on the easiest way of how to use it.