All you need to do is add the language pack and use the language bar to switch to Korean.
I’m on Windows 7 Home Premium and I can write in Korean just fine. I think the other people who answered think that you want to see the all the text in Windows in Korean language (which is what the Ultimate feature does, I believe).
1. Go to start and go to ‘Region and Language’
2. Go to the ‘Keyboards and Language’ tab
3. On the ‘General’ tab click ‘Add’.
4. Navigate to Korean and select Korean and Microsoft IME options
5. Click OK, you should have those options in your list now
6. Go to the Language bar tab in the same panel, and check ‘Docked in the Taskbar’ (that’s where I have it)
7. The language bar lets you select Korean language by clicking on the EN, then switch to KO
8. Change the keyboard input then by clicking on the capital A and changing it to 가
9. You should be typing in Korean now!
10. If you load the ‘Onscreen keyboard’ you should be able to see where the keys are and what your keyboard is set to.