Since a great number of people have asked on how to enable the Chinese Character Input function that's built into Windows XP, I'll post a comprehensive step-by-step tutorial right here.
What you need: Windows XP installation disk, your computer (duh)
Step 1: Insert Windows XP installation disk into your CD-ROM
Step 2: Click on the Start Menu

Step 3: Click on Control Panel

Step 4: Click on Regional and Language Options once the Control Panel opens

Step 5: Now you should see a dialogue box, click on the Languages Tab

Step 6: Make sure that you check the "Install Files For East Asian Languages" box, then click "Ok" for the dialogue box that pops up. Then click "Apply"
Now your computer should be installing the new language files. This takes up aroud 250 MB of your hard drive space, make sure you have enough.
I think after it installs it requires you to restart your computer (not sure), but if it does, restart. Then folow steps 1-4 again and click on the Languages Tab, so you're at the screen in Step 5.
Step 7: Now check to make sure that your "Install Files for East Asian Languages" box is really checked, and then click on "Details" button, it should bring up this window:

Step 8: Click on "Add..." button, it should bring up this window:

Step 9: Select "Chinese (PRC)" for Input Language, and the Keyboard layout/IME should be automatically set to "Chinese (simplified) - US Keyboard", if it's not, set it to that.

Now click "Ok".
Step 10: Now you should be seeing the following screen:

Step 11: Click on the "Key Settings..." button, and it should bring up this box:

Check to make sure that the "Switch between input languages" action is set to "Left Alt + Shift", it should be. Then click "Ok".
Step 12: Now click "Ok" for all your dialogue boxes so you close them all.
Warning: Do NOT click cancel, as this will ruin all the previous steps!Now in your menu bar, you should see the following:

Step 13: If you press left-Alt and Shift together, the little "EN" should change to "CH"

Step 14: Now you can input Chinese characters by typing piyin!

Enjoy! If you have any questions, feel free to post them here, or just PM me directly.
3nodding