It is unlikely that you have Giardia in a deep well that is not under the influence of surface water. It is even less likely that some guy, without extensive testing, is going to be able to tell you that your well is probably contaminated with Giardia.
Giardia lamblia is a cyst that is sometimes found in surface waters. The only wells that are considered susceptible to Giardia are shallow wells and springs that are subject to surface water infiltration.
Such wells are classified as "wells under the influence of surface waters".
The problem with testing for Giardia is that testing is not conclusive. It can be absent one day and present later. The usual test is to do a culture test for organisms that are found in surface waters. If such organisms are found, then the well is classified as "under the influence of surface waters" and the treatment must be as if it is a surface water supply.
The EPA requires filtration for public water supplies using surface waters. Disinfection is not considered reliable for killing the larger surface-water organisms such as Giardia and Cryptosporidium.
http://chppm-www.apgea.army.mil/WPD/Disinfectants.aspx
UV is considered acceptable for individual water treatment systems. The water must be pre-filtered to keep the lamp from being coated with substances that will prevent the light from killing the organisms.
If I knew or suspected that my well was contaminated with Giardia, then I would probably filter and chlorinate. If it has organisms related to giardia, then I would want a residual disinfection, such as chlorine.