I was a little surprised when a dear friend told me she didn't use anti-virus software.
Why? Not having anti-virus software is harmful to both you and everyone else in your address book. Not having anti-spyware hurts your computer's speed and records information about your computer activities. And you don't have to buy it.
It is not OK to do without anti-virus software. You can get free versions (for non-commercial use) that are great and don't bog down your system like paid versions such as Norton do.
Free anti-virus software
Manual, license, instructions, all free
I use Ad-Aware for my anti-spyware client, and I run it once a week.
If you don't have anti-virus software . . . .
Don't leave your chair until you've downloaded and installed it.