Applications Start automatically , Start again if I Force Stop them or even if I Disable them Many applications like skype , orkut, yahoo msger etc start automatically... i have already disabled and force stooped them .... I don't want to run these app and others like WiFi sleep etc to save my RAM ... I have already used Application to kill running application like Task Killer, RAM Booster etc .. but they also have limits and are unable to kill app which are installed when a new firmware is installed like Skype Yahoo msnger etc...
When installing you granted all of these apps an explicit permission to start when your application starts!

But for the examples you give (though I haven't ever installed Orkut myself), most of these apps offer settings internally that allows you to choose whether they start automatically. 

(Recent versions of Skype have disabled launch-on-boot, have you tried updating your version?)