How do you start up from disk
My 2007 iMac has become very sluggish. The little colorful round ball justs spins and spins so much now. I went to Disk Utility and for the first time selected verify disk. Part way through the verification I got an error message telling me "The volume Macintosh HD was found corrupt and needs to be repaired.", "Error: this disk needs to be repaired. Start up y...isc), and then use Disk Utility to repair this disk." The problem apparently comes from "invalid volume file count" and "invalid volume directory count".
I am sure this is a simple task for most but I am unsure how to proceed. I have my original OS X Leopard disk and my upgraded OS X Snow Leopard disc. Any direction in how to proceed would be greatly appreciated.
2010 Mac Mini with 4GB RAM and iMac 20 inch late 2007 1GB RAM 250 GB HD, Mac OS X (10.6.6), Time Capsule, iTunes 10.2.1