What computer? Brand, model, age, anything else you can think of....
OS=Unknown? Now who would not know what Operating System his/her computer is using?
It's on the screen every time you boot up.
If it's a store bought computer, there is probably a license key sticker on the side or back and that will have the Operating System printed on it.
It's really hard to answer questions about something without any information about it.
Heck, it might be a computer and again, it might be a riding lawn mower.
More info please!