I'm not sure about Windows 7, but in Windows XP you have to install Asian Language support for Windows.
Control Panel -> Regional and Language Settings
You need to hit the checkbox for East Asian Language support.
After that, it gets a bit tricky in terms of understanding how to pull up the language bar.
Once you've gotten language bar up
In general, I think the main toggle is CTRL-Space (and sometimes you need to hit the Left-Shift button to adjust between typing Chinese and English.
Once you've got that set up, you can use the other input methods, including Sogou and Google. I prefer the Google one, but I've heard they are very similar.
Aside from that, we're generally assumes you're using pinyin 拼音 to write characters.
If you're using linux, I think you can install SCIM (smart common input method) system. It should be easy to find, depending on your linux distribution.
Here's another answer with this