Don't use them in instead of, but in addition too.
Doing spyware scans from several different spyware detection programs has the potential to find more spyware. I had the situation of one program finding a worm, another finding something like a keylogger, and a third a backdoor trojan. Likewise for Virus scans: I had two programs that said I was clean, while another found six related virus files.
Note: when running one of the other virus scans, be sure to shut down the active scanning of your anti-virus program to prevent conflicts from happening.
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