No legitimate person responsible for security or systems maintenance needs to know your password, and there are almost no exceptions to this rule. Don't trust remotely obtained software. It can contain Trojan Horses that are potentially devastating in their effect. Autoinstall allows remote Web sites to automatically change what your system does by installing their software. Check them periodically and whenever you suspect or observe abnormal system behavior.
More viruses spread occurs today as a result of email attachments than anything else.
Be careful how you use email attachments and who you accept them from. When I don't know and trust the person sending me an email attachment, I either ask them to send it in plain text format and not as an attachment, ask them to FAX it to me, or copy it off of my system onto a non-networked system and read it there. Otherwise, you can easily be attacked by an email. Don't trust excel spreadsheets. Many programs innocently do things that weaken your security, and in lots of cases they allow remote exploits against your system.