Buggy iMac

What would cause an iMac to act as if it were infected with Windows malware, when it's never been run in a Windows environment?

Multiple crashes, constant freeze-ups, even ejecting a CD unprompted.

I can't even scan for malware using iAntivirus; it crashes before you can complete the scan.

It's unlikely you have malware and certainly don't have a virus, so the first thing I'd suggest is booting from your bootable backup (if you have one) or your system restore DVD (if you don't) and see if the problem continues.

If it does, it's a hardware issue.

If it doesn't, reboot as normal and try to create a new (admin level) user. Then log out of your account and into the new user. Does the problem persist, or go away?