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 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. 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.