Install the drivers.
Are you sure it's an Uno R3? Many clones are sold as "Uno R3" clones, but actually use a different serial chip. Check the markings on the black chip nearest the USB connector - it'll either be an atmega 16u2 (use official uno drivers), or a CH340G (use the CH341SER drivers, file's called ch341ser.exe - google), FT232 (use FTDI drivers, included with at least recent versions of the IDE), or something else (google the part number).
Why are you using an ancient version of the IDE, instead of a 1.6.x version? (I recommend 1.6.5r5 - or if you're stuck with 1.0.x versions, use 1.0.6)