Q: Screensaver password forgotten - is there anything I can do?

http://support.apple.com/kb/HT1274 if you have Mac OS X 10.6.8 or earlier.

http://support.apple.com/kb/HT6022 if you have Mac OS X 10.7 or later.

Note PPC Macs date from early 2006 and earlier, and can't run anything newer than 10.5.8, so would be included in the first link.

To verify your model iMac, see this tip:

https://discussions.apple.com/docs/DOC-2672