The majority of spam e-mail does not carry threats, and those which do are most often targeted at Windows computers, not Macs. You can help us see if you have any malware, but not when it was acquired. Please download and install this free utility:
http://www.etresoft.com/etrecheck
It is secure and written by one of our most valued members to allow users to show details of their computer's configuration in Apple Support Communities without revealing any sensitive personal data.
Run the program and click the "Copy report to clipboard" button when it displays the results. Then return here and paste the report into a response to your initial post. It can often show if any harmful files/programs are dragging down your performance.
The program will also tell us what version of the iMac you have. This forum is for pre-2006 iMacs and most people post here by mistake because of Apple's insistence on using non-intuitive forum labels. If that happened, we can get you moved to a more active forum without your having to retype the question.