iMac crashes when I access Safari

Sounds like you have a corrupted file.

Do you have a backup of all your apps & data? You'll need to do that first.

Do you have a USB thumb drive or FireWire/USB HD which you can prep with a fresh copy of OS-X and make bootable? If you do start there as you'll need to have your external drive boot up so your internal drive is not being accessed (no open files).

Then using the copy of Disk Utility on the external drive run repair permissions first then run repair disk and then permissions one more time. See if that solves the problem. If not go to next step.

Create a new user account on your system then log in under that user. Launch Safari is it working? If so we now know Safaris' settings & cache files are damaged.

This could be malware. Do you have an anti-virus application? If so make sure you have the lated version and run it did it find anything? If not then you'll need to go into the users library folder and navigate to the Safari directory and manually delete things. This is a bit tricky for now. Lets see if you don't need to get to this point. I have to dig up my notes on how to go about this so I'll come back with the info if you need it.