I used to use commercial antivirus products like McAfee and Norton and Trend Micro etc. I went with Avast Home for free. I scan with Emisoft Emergency Kit because it can fit on a USB drive and be portable.

No matter what you do eventually you will get infected by some exploit. Be it Flash, Java, some pop-up ad, or just an email sent to your email client that exploits it.

I got a lawyer still runs XP and Vista and uses ClamAV because it is free. I worry that her systems might be infected, but her husband runs the tech support for their firm. She used to have employees steal data via floppy disks, and these days a simple virus infection can steal data.