There are many good, free AV applications for the computer user (see the link):http://www.bleepingcomputer.com/forums/topic3616.html
I have used Avast and AVG, but the former at least on my computer with its configuration, was slower than AVG and used more resources. Other Members have had good experiences with Avast.
However, it is best to have only one resident AV on your hard drive, especially if both attempt to provide real-time protection. Running two concurrently can not only slow down your computer, but produce serious conflicts. Just as importantly, it can be confusing when one AV discovers the virus definitions of the other and wants to quarantine them.
There is always the option of using one of the many good on-line AV scans to double-check the protection your own AV is providing.