Customise a SharePoint List Form
There's no point reinventing the wheel. Rather than sitting down myself to create a demo of this wonderful new feature you can get by combining SharePoint 2010 and InfoPath 2010, I thought I'd link you to this video put together by the InfoPath 2010 team.
They plan to make a whole series of these videos so if you have an interest in InfoPath demos, I suggest subscribing to the team's blog.