I don't use McAfee VirusScan, but it has settings just like any other anti-virus program that can be changed to suit your needs.
You apparently have it set to scan during startup or scan after startup. Browse around in the tools/options/preferences settings until you find the setting(s) that need to be changed.
Personally, I keep automatically scanning turned off because I like to have complete control over when my anti-virus program runs a scan.
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