How can I determine if my MBP running Snow Leopard has been attacked by a virus? 5 yr old MBP Intel bought for video editing ran fine for 4 yrs, using Tiger &amp; FCE 3.5.  Past year has been stormy.  Onset was unrecoverable crash dx'ed as not-worth-fixing logic board failure by genius at Mac Store.

Community-based Apple tech repaired logic board, which made computer bootable, but  occasional crashes recurred.  Additional servicing was done:  new 250GB hard drive, upgrade to Mountain Lion, followed by partial downgrade to Snow Leopard (clean install) to restore compatibility with FCE 3.5. 

System seemed to be working but multiple worsening crashes began yesterday after transferring ~ 3 GB iTunes tracks from old Tiger CCC bootable clone.  Currently running an Apple Hardware Test from Tiger install disc.  But viruses apparently do attack Macs and I would like to know the best way to determine if my MBP is infected.

Any advice would be appreciated.  This is my first Mac, and I'd much rather be editing video than troubleshooting!
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"