You would have to have a Chinese version of windows in order to install a Chinese program in many cases.
I used to run Engish windows, and as you said, I was able to view Japanese websites, and input Japanese characters, but I couldn't install any Japanse software into my comptuer without the characters getting all messed up.
The good news is, the opposite is NOT true. If you buy a Chinese version of windows, all English software works no problem.
I have heard that Windows Vista will allow us to select what language version we want to run, and with the click of a tab, switch from language to language (like you can with a Mac). I hope this is true!