Here's the specs on your machine: http://everymac.com/systems/apple/ibook/specs/ibook_g4_1.42_14.html
It originally came with system X 10.4.2 (8D37)
It will take up to system 10.5.8. You may be able to get 10.4 direct from Apple for $16. Give them a call with your serial number and ask. This being to costly, try ebay. When you boot from the CD, you can go to the second screen pull down menu to Utilities > Change password, and just get rid of the old password.