new hardware disables virus program

I tried to amplify the signal on my Dell router by installing a D-Link
"Wireless Range Extender" on the client computer with a Dell wireless
adapter. In order to do this I had to install my SMC ethernet card to
connect the D-Link extender. But as soon as the ethernet card was
detected by WinXP, my McAfee virus scan was disabled. I went into safe
mode and did a virus scan, which turned up nothing. I removed the card,
and the virus scan was automatically enabled, as usual. I disconnected
the wirless adapter and repeated this process, reinstalling the card,
and the same thing happened. So now I have removed the card. But how
could adding an ethernet card disable the virus program? I'm confused.
Howard Kainz