I agree with Xiao Hu on 搜狗 (sougou) being the best one. Just click the link and click the big green button that says 立即下载 (download now). When it downloads just install it and keep clicking the 下 button.

Afterward, push the Windows and R key (or start ->) and type in intl.cpl to bring up the language bar settings. Click keyboards and languages. Click 'change keyboard'.

In here you can add a keyboard, in your case you'll want Chinese. The one you want should look something like 中文（简体） - 搜狗。。。。

Then click over a tab to language bar. Set it to float or on the taskbar. Your preference. Then after that just close it up.

When you hit left alt and left shift it'll shift between languages. You can see the bar appear in the bottom right corner or wherever you docked it. Then you can type away in pinyin! Wohoo!

Let me know if you have questions :).