Despite all advice, they've been running together for at least two years
and no trouble seems to have arisen.
Glad to hear it, but that's like saying "I've been driving without a
seatbelt for at least two years and no trouble seems to have
Running two anti-virus programs at once doesn't guarantee a problem;
it increases the risk of having a problem. Increasing that risk is
*not* the thing to do.
And it adversely affects your performance.
And finally, as others here have said, despite their popularity, these
are the two poorest anti-virus programs available. I hate to agree
with Alias, but in this case I do--dump them both and install *one*
good anti-virus program.
I recommend eSet NOD32 or Kaspersky, if you are willing to pay for it.
If you want a free anti-virus, I recommend one of the following three:
Microsoft Security Essentials
You also need anti-spyware software (even if you run a program like
Microsoft Security Essentials, with anti-spyware capability built into
it). I recommend that you download and
install (freeware) MalwareBytes AntiMalware