Good antivirus software is a necessity. Without it, you run the risk of having your computer damaged by malicious software. Your data could be erased or Windows could be rendered inoperable.
There are many reputable companies that sell antivirus programs. But you don't need to pay a dime for antivirus software.
PC Tools is one of my advertisers. It is offering a free edition of its AntiVirus program. You'll also get free updates periodically with new virus definitions.
The only drawback to free antivirus programs is that they don't include technical support. If you need technical support, you should bite the bullet and buy and antivirus program. PC Tools also offers a for-pay version of its software that includes technical support.