Even with virus checkers installed on your machine you are at risk at picking up a virus on your PC / Laptop so therefore follow the golden rules of updating windows (ideally have the most recent version of windows on your machine) install and update your virus checkers and firewalls and never open emails or attachments from people you dont know (and be aware that people can use other peoples computers to send emails from their accounts pretending to be them).
This example statement from a website client shows the problems a virus can cause.
I was awaiting some reply to my last email regarding beavers website. I see that the client maybe providing some difficulty in regards to the website. From my prospective we have fulfilled the brief but cannot access my email, as my defunct computer vista operating system decided it was going to become infected with a “virus” and make my desktop completely unworkable. After a day of going though regedit to remedy the problem I gave up and have reinstalled to factory condition. I am completely gutted again. I have Norton internet security, firewall, and firewall on the router with passwords. I tried a trojan remover but none of them worked. I tried some DOS stuff to no avail. Upshot is for me I’ve lost all my passwords that where stored in my browser hence I cant access my domain email account.