See the Next Step section.
The Users Tip was written some while ago and the links about checking the Servers where you got your name from are out of Date. (First section)
For AIM names you can Start here
You can use the Sign In option top left then click the Forgot Password button
(Hopefully you set up an alternative email and security questions to help you get the password)
Use iForgot for MobileMe and @mac.com names.
For Google names you should login in to Google or at least try and follow their Password Recovery route.
For other Jabber names you may struggle as many don't have Web Site logins that you can use.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.7),
"Limit the Logs to the Bits above Binary Images." No, Seriously