If I'm going to return my computer to the store for a replacement or refund, what do I need to do to clear out any passwords or history from being visible to workers who may have access to the computer?

I bought an Acer netbook around the holidays (so around 4 months ago). I bought it from Costco, and they double the manufacturer’s warranty, so it has a 2 year warranty on it (which is one of the reasons I love Costco). The “H” key is a little stuck, so I have to press a little harder on it, or else “H” is not typed. I know this problem is not going to get better with age, so I may as well return the computer asap and get a new one or a refund.

What do I need to do to prevent my personal information (passwords, sites I’ve visited, credit card info that’s stored on shopping sites) from being accessed by people who may fiddle around and turn the computer on?