If the grout is dirty but in good shape then I wouldn't try and regrout. In order to regrout at least half the thickness at minimum of the grout must be removed. This is so the new grout can attach it'self to the sides of the tile.
Instead just clean it up using a rag and a small brush together with a product called Bon Ami. You can find this in the cleaning isle at your grocery store and it comes in a cardboard container similar to Coment. I mix it into a paste using a drop or two of bleach and it always cleans the grout up real nice. Here's a link to show you what it looks like.
http://www.faultless.com/Products.as...n+Ami+Cleanser
Then when clean and thoroughly dry I apply a quality sealer such as Sealers Choice Gold. It has a claim of lasting for 10 years though harsh cleaners cannot be used on it or it'll strip away. The back of the bottle will tell you what to use to clean it so it'll last longer. Here's a link to it:
http://www.aquamix.com