Can I go from Yosemite back to 10.6.2?

[H0TH]

My 2009 iMac is starting to go slow and lock up, I've got my install disk that came with the machine, but when I try to install it, I'm told it's too old. I've followed some tutorials that tell me how to start over, but I get a message about not being able to unmount my HDD
Oh, and like a genius, I don't have a time machine backup from before Yosemite

 

[gsahli]

Start up using the DVD. Use Disk Utility to erase and re-partition the drive to 1 partition. Then install.
You do know you'll lose everything on the drive this way - right?
You should back up before starting.

 

[Lifeisabeach]

Has the iMac ever worked properly with Yosemite? If it used to, then it would be in your best interests to figure out WHY you have developed problems. Simply reverting to an older and increasingly dated version of OS X isn't necessarily going to help, especially if the problem is a hard drive that is too-full, or failing.