I ran Techtool 4.6.1 on my G4 desktop, specifically to optimize the drive. Some repairs couldn't be made when I ran the volume structures diagnostics. I was pretty much finished at that point without troubleshooting but now the 'puter gives me a message "You must restart your computer... etc. and nothing I've done so far will start it again. I can't get past that message.

This appears to be a problem with Leopard and Techtool 4.6.1, it has been reported on Macfixit.com (http://www.macfixit.com/article.php?story=20071212101326564) and else ware. I have not experienced any problem with any of the clients I support. Then I use a different utility. You can try to uninstall Techtool 4.6.1. To uninstall an application from a computer you can not boot you can try to boot in safe made here are the instructions (http://support.apple.com/kb/HT1455). If this will not work you can try booting to another system drive or T booting and connecting to another Mac running the same processor architecture PPC to PPC or Intel to Intel and removing the program.