Windows Vista is the first Windows version with a Tajik keyboard, based on the Russian layout. I made a Tajik phonetic keyboard layout for Mieke, which you can download here. Extract the zip file and open setup.exe. The keyboard will install itself as Uzbek because Tajik is not available on Windows XP. See the picture below for the layout.
Not all Windows fonts contain all Tajik letters. It is best to either use Palatino Linotype (included with Microsoft Office) or the free Charis SIL font.