Don't worry about it. Hard advice I know, but google is just as desperate as everyone else for quality candidates. Get some sleep, drink some coffee before your interview, and just talk through the problems as you get to them. If this is your first interview it's likely a phone interview. They usually last about an hour, and they just want to make sure you can think through a problem.

Also, regardless of whether it works or not, don't let it affect you. I know some really brilliant people who couldn't get into google, but are masterful coders.