Any way to fix my iMac?

So a while ago I installed Windows 7 to a hand-me-down iMac I got from my aunt, and everything was fine for about a month and a half. Then I tried to switch to the Mac side one day, and it froze at the apple boot screen. I tried everything to fix it, but it didn't work. Then I tried wiping out the Mac drive, and using an install disk. However, it's 10.6, and from what I've seen I'd need 10.5 for mine, and so Snow Leopard "can't be installed" on this iMac. I have a Mac OSX .dmg file on my windows drive, is there anyway I can install it from there? Or am I out of luck without burning it to a DLDVD, or finding the original install disk?