Suppose my iMac doenst have a DVD drive, and i have the SnowLeopard bootable dvd disk, what do i do then?For Snow Leopard - you would boot to your Snow Leopard installer DVD, and erase/reinstall with that. (there is no internet install for Snow Leopard, as with newer OS X versions)
If you don't have the DVD - the Snow Leopard installer is still available for sale from Apple.
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard