Keychain problems...

Hi, I have just bought a second hand Mac Mini. I've changed the user stuff around, and connected it to my wireless network. Every time I open a different web page, Safari keeps asking me for the Keychain password. Unfortunately I don't know this, and can't seem to find a way of changing it without knowing what it is originally.

Is there anything I can do to change it short of reinstalling everything?

Any help is much appreciated.

Thanks, Paul