no harm in installing 2 anti-virus softwares at the same time
just ensure that one has
only an on-demand scanner (ClamWin) while the other has an on-access scanner (Avast/AVG/KAV/NOD32)
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edit
Was it just me or has Norton become so unpopular that it dint even come across my mind while posting
(i had some other AVs also in mind while posting, but did not add them)