Kees - thanks for responding to my post. I am unsure about how to interpret your statement "A worm is a program, and you are supposed to know what programs you installed into what folder and when you did it."
According to Panda Software "Netsky.P is a worm that deletes the entries that belong to several worms, including Mydoom.A, Mydoom.B, Mimail.T and several variants of Bagle."
"Netsky.P spreads via e-mail in a message with variable characteristics, and through peer-to-peer (P2P) file sharing programs."
"Netsky.P is automatically activated when the e-mail message is viewed through Outlook's Preview Pane. It does this by exploiting a vulnerability in Internet Explorer, which allows e-mail attachments to be automatically run. This vulnerability exploit is known as Exploit/iFrame."
"Netsky.P is difficult to recognize, as it does not show any messages or warnings that indicate it has reached the computer."
I don't believe that I (as you suggest) installed Netsky into my system. In fact, when I subsequently ran the Pandasoft removal tool, like the other removal programs that I had run previously, it detected no trace of Netsky. False positive I think.