How can I determine if my MBP running Snow Leopard has been attacked by a virus?
Apple Hardware Test will not test for virus, try some anti-virus scanner from reputational software provider like free anti-virus for mac. But remember all anti-virus can only check for known virus or bug, small or custom virus cannot be detect.
If you have extensive knowledge about computer you can open "terminal" and type "top" to see all the computer process and look for the process that take up all the memories, then exit with "contrl+z"