Service tax on software sale

D/Sir We are I.T.Company and selling two types of softwares to our customers. One the canned (readymade) software i.e. not made for particular customer but can be used any one globally. And other the un-canned software - this software we developed as per particular customer's need and requirements. On selling transaction of canned (readymade) software we charge only M.VAT / CST and not service tax and on un-canned software we charge service tax as well as VAT / CST. So pl advice whether we are doing right thing? Thanks Arun