a.) If proper precautions weren’t taken to remove the worm from any servers the computer connects to, and then as soon as the computer connects to the reconnected server it could be re-infected again.
b.) It’s possible that another piece of malware reinstalls the files upon reboot.
c.)The other possibility is that you had not in fact removed all pc files that were infected with the worm.
This was a team question, I, Kat, Jake, and Ellen answered this one for you.
On a side note: Help Net Security does a decent job of writing a monthly top ten list of malware threats.