During home inspections,tile and grout problems are fairly common. Leaving your deteriorated grout can cause broken tiles, and damage the material behind the tiles especially in the shower or other wet areas.
This video gives a comprehensive step by step on some ways that you can fix that old or missing grout. I like this video particularly because not only does it go over removing grout with a hand tool, but also touches on a way to deal with larger jobs using and electric tool. The electric tool that he is using can be purchased from $80 to $200. In fact I just purchased the one shown hear. I did have a no name brand one before, however the Rockwell version is much more impressive. When you are looking at these, generally the higher the amp rating, the better. The spacific
On to the how to re-grout tiles video:
If you are doing this kinda work all the time you would be better off with the diamond blade, however if you are just doing one job you will be good with the carbide blade.