You ought to try Cineform's Neoscene, your computer is underpowered for processing native AVCHD, and you should use a intermediate codec, your computer specs are just over the minimum requirements for Neoscene, you can download a trial, if you like it you can purchase a key to unlock it. you can buy it from a internet vender like Videoguys, Safeharbor, etc for about $30 cheaper.
http://www.cineform.com/neoscene/