I use ESET anti-virus on ten different machines of varying ages and configurations. It doesn't seem to impact performance.
As an IT consultant/installer to very small businesses of up to 10PCs, ESET Security Essentials is my recommendation and seldom ignored even if free trials are pre-installed on new equipment. Got a configuration or corruption problem caused by self inducement then ESET free instant telephone support is absolutely top notch. Try even finding the telephone number of the competition!
You have the option to try before you buy, if you so wish, and if you have several PCs to protect, then additional licences are discounted. The reviews of the product says it all.