It won't be a virus causing your problem - there are currently no viruses 'in the wild' that affect OS X. Can you elaborate on what the problem is? Activity Monitor in the Utilities folder should show what processes are running and what is gobbling up your RAM.
Also, how much RAM do you have installed on your computer? Going to the Apple Menu and selecting 'About this Mac' will show the amount installed. Lack of RAM could be a cause of slowdowns.
It's also possible that you have some incompatible software causing the problem, or even some software that is known to cause havoc with otherwise healthy Macs. You could download and run EtreCheck and post the contents of its report here - it may reveal potential causes:
http://www.etresoft.com/etrecheck