How do viruses come in

[Trader J <tradetudrop@xxxxxxxxx>]

My computer was scanned last night (and every night)
for viruses.  None found.
This morning, I had my Earthlink internet connected at
46.6 bps, but with IE browser off.
I had IB TWS on.
I had TS2000i with eSignal Turbofeed Data Manager and
OnDemand Server on.
I was watching the market.

Less than an hour later, my Norton Antivirus found 3
"backdoor" viruses.
Could someone knowledgeable in viruses explain what
happened?

Thank you,

Julian