I would say most consumer ISP's provide pretty bad DNS service, and you can clearly tell that they are running slow when you spent a lot of time waiting for the browser to finish "resolving name www.xyz.com".

I am constantly surprised with how big of a difference it makes when I am working on other people's machines.

Here is their page with instructions on how to change yours.
http://code.google.com/speed/public-dns/docs/using.html

One alternative is opendns (if you don't like google).
Their service is good and even has a few features that google doesn't provide, but you are required to make an account to turn off their more annoying "features" that are on by default.

However, their help page does have extensive instructions that includes *pictures* of what needs to be done, which makes it really easy to figure out where you need to go

https://store.opendns.com/setup/computer/

You can use their instructions and replace their numbers with google's if you like - the steps are exactly the same.

Both have their own pro and con's (especially if you look at them politically) but google's servers are the least hassle to setup.