It would be easier if you tell us what the specs are for your computer and we can tell you if Reflex will run on it.
You can buy Reflex at [link=http://ecsvr.com/mta/shopdisplayproducts.asp?id=17&cat=Simulators]MTA Hobbies[/link] or you should be able to get it at most of the big on-line hobby shops.