SSL doesn't encrypt your data when it is still on your computer.
It only encrypts the data while it is traveling to the other party,
for example, the bank.
Most sniffers work because they try to sniff the packets while they
are being transmitted, so SSL defeats that.
But if a man-in-the-middle attack happens, it's as good as having
no encryption. That's because such attacks first route the packets
to the attacker, then to the intended party, pretending as if
nothing has happened. In this case, all packets are not encrypted
and will be in the clear.
