2008 iMac Running OS X 10.8.2 Won't Boot
The graphical "noise" makes me think that either the graphics drivers have been corrupted (and by the sounds of it, likely more OS components), or there's a hardware defect.


Does the "noise" appear when the Apple logo is on screen, or just afterwards? If during the Apple logo, it's more likely a hardware problem, since that boot screen isn't based on the OS install. If afterwards, it may just be a problem with your install.
You can try booting off the Recovery Partition by holding down command-R when you boot.

If this goes well, then it's likely a problem with your boot partition being corrupted. The recovery partition includes Disk Utility, which will let you run a basic check of your boot partition, which may or may not be able to fix things.
If this fails, then you either have a hard drive issue, or some other hardware problem, in which case you're almost certainly going to need to consult an Apple Store or Service Provider.

You can also attempt to boot off an external drive as segiddins suggested in the comments.


Sorry I can't be definitive, but it's hard to say for sure without having the machine in front of me. Try some of these and let us know how it goes, and we can help narrow some things down for you.