frustration

I can't turn off my imac without pushing the switch at the back, I click on shut down am asked if I want to shut down immediately and do so, then I lose the top bar and the task bar and nothing, no spinning wheel to tell me anything is going on, no little clock lines disappearing in clockwise order, nothing.
can anyone help

The first thing I would do is boot up from the 10.5 Install DVD and from the tools menu run Disk Utility. Have it first repair permissions then check and repair the Hard Drive. Report back if it helps or not if you would.

You can do the permissions repair from the Disk Utility right in Leopard, but you can not repair the drive while OSX is loaded into memory, and that is why I suggest booting from the OSX DVD.

Can you just talk me through the technicalities of what to do so that I can print them out and do it properly, the install dvd is sitting on my desk so it shouldn't take too long
thanks

Insert the DVD for your Leopard OS. Restart and old down the "C" to start from the DVD as your startup drive because you cannot repair a drive you are using as your startup drive hence using the DVD.

When the DVD starts up (on the right hand side the DVD will appear above your HD icon) I believe one of the options on the title bar is Utilities and select Disk Utilities. Report back any errors not "fixed". When done just restart and your should be good to go. Eject the DVD and let us know how you made out.