How can I write in Pinyin?

You don't need a special keyboard. Microsoft word has all the special letters you need. In windows microsoft word, all you need to do is go to "insert symbol" and then click the symbol You want to create a key for, then press button that says "shortcut key" this will allow you to make your own shortcut on the keyboard to type in any symbol you wish. For example, you can set your keyboard to place an "ǎ" in a word by pressing ctrl and 3 at the same time, ( for third tone) then press "a" and voila! a "ǎ"! However, this only works inside of the Word processor program. and you have to program each five vowels and four tones separately. You may just have to cut and paste from there. I know it sucks, but I have yet to find a pinyin key board either. Let me know how creating the pinyin key board layout goes! Good luck on learning chinese! (I have taken it for four years and still love it!) :D