IMac (Intel) :: Still Running Slow And Crashing After Malware Removal?

Last week I got a malware attack from opening up a Facebook message. This caused my iMac to run very slow and to crash all the time. I ran the latest security update which found and removed the malware itself. But still my iMac is running very slow and crashing when doing two jobs at once.  

Any reason as to why?Is it still infected? or has the malware corrupted my hd or memory? 

My iMac details: 

Version 10.6.8 
Processor 3.06
Memory 4gb 

Info:
iMac, Mac OS X (10.6.8)