There's really no way to tell. You can only find out what the current OS is installed, from with in Windows itself. You could do some detective work. Check the PC's vendors website, cross reference make/model number and see what original OS was installed by checking what drivers are avaiable for what OS and compare that to what OS you've and kind of guess how many in-between was installed. The only other way, would be a paper trail. If you kept all your OS purchases, you'd know.