Mac OS X 10.7.4. How do I configure my network settings so I don't have to enter the password to my wireless router every time I start up or come back from sleep mode?

When I first got my MacBook Pro my wireless network at home (Linksys Wireless Router) popped right up and I entered the password and had no problems automatically connected after that. I went to a friends house this weekend who also has a wireless router named Linksys, obviously with a different password. Ever since I connected to her network I have to enter my network password at home everytime I start my computer up or wake it up from sleep mode.

My questions is what caused this to happen and how do I reconfigure my network settings so that I don't have to re-enter my password all the time?

Thanks