Can i use dell os disk on acer

No.

yes but if its not a dell you have to have a product key to use. But otherwise it would work. because its not a dell it will not automatically activate windows.        

Most Dell Windows CDs check for a Dell BIOS.

Most major system builders have their own series of produce key, Windows OEM system cd's that check the BIOS their branding.

If you try to use their cd on another system and COA windows will not install