If you have installed the appropriate security updates then you computer is protected. See
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.
I am visiting my elderly mother and (like an idiot) responded to a prompt to update Adobe Flash last night. I have checked for the presence of DYLD_INSERT_LIBRARIES per C|Net's article on how to detect and remove the Flashback malware and it is not present in Mac OSX, Safari or Firefox.
Flashback has been extinct for at probably nine months now and there have been no new infections for over a year, according to all the A-V labs.
I will read up on the malware detection and make ure that all is well!
Slow-downs are almost never the result of malware, and as others have said if OS X 10.6.8 is fully up-to-date it is protected against all currently known malware that can impact it.