This all assumes you are on Windows XP (though I think it’s the same, more or less, for older versions of Windows). Note that some of the names of menus, tabs, or menu options might be a bit off, ’cause I just took the Danish names and translated them literally, rather than take them from an English Windows environment. Just choose the one that seems most like what I write.
If you see a small blue square with a two-letter representation of a country/language on the right-hand side of your menu bar (the one with the ‘Start’ button on the left), do as explained here, otherwise look further below, to the steps labelled 1a, 2a, 3a, etc.
1) Right-click on this blue box, and choose ‘Settings’.
2) In the dialogue box that pops up, click on ‘Add’.
3) Choose the language you want to be able to write in from the drop-down list of languages. Click OK.
4) [not required, but a good idea] For easier and quicker switching between languages, click on the button that says ‘Shortcut settings’ or ‘Key settings’ or something like that. Higlight the language you use normally and click ‘Change shortcut’. Choose a combination of keys that you want as the shortcut for your standard keyboard layout (I usually use Ctrl+Shift+1 for Danish). Click OK. Do the same with whatever other languages you’ve added to the list—choosing, for example, Ctrl+Shift+4 for Spanish and Alt+Shift+2 for Lithuanian (I wouldn’t personally mix the Ctrl+Shift shortcuts and the Alt+Shift shortcuts—I’d only end up confusing myself—but that’s up to you). Once you’ve done this and clicked OK a few times to get out of all the menus and dialogue boxes, you can now just click Ctrl+Shift+4 anywhere in Windows (in any program), and your keyboard will turn into a Spanish keyboard (if you used Ctrl+Shift+4 for Spanish, like I did in the example above. When you’re done typing in Spanish, just click Ctrl+Shift+1 or whatever other shortcut you chose for your ‘regular’ typing language.
If you don’t have the little blue box with the ‘language initials’ in your menu bar, do this first:
1a) Go to Start -> Control Panel, and set the Control Panel to Classic Layout, if this hasn’t been done already (the first of the ‘suggestions’ Windows has on the left side).
2a) Go to ‘International and Language Settings’ (or something similar), and click on the second tab (‘Languages’) in the dialogue box that pops up.
3a) Click on ‘Details’ (or ‘More’ or whatever it’s called). This brings up the same dialogue box that you get by doing step 1 in the above description.
4a) Click on ‘Language Toolbar’ (bottom left), and tick off options one and three (‘Show language toolbar in the menu bar’ and ‘Show extra options for language toolbar in the menu bar’, respectively), and click OK.
5a) Continue with steps 2, 3, and (if you wish) 4 from the list above.
When you click OK in all the boxes you have on your screen after doing this, you can change your keyboard back and forth between all the keyboard layouts you’ve added by choosing them in the list that pops up when you (left-)click the little blue ‘language initials’ box in your menu bar. If you want, you can go back and add more languages to the list in exactly the same way as you’ve done just now.
If you choose not to do step 4, you can of course still change your keyboard layout by clicking on the little blue box in your menu bar and choosing the appropriate language; step 4 just assigns specific shortcuts for individual language layouts.
If you want to enable more complex languages (Thai, Chinese, Korean, Japanese, etc.), you need to do steps 1a and 2a, and then tick off the two options under the ‘Details’ button. They both say something about installing support for complex languages or something. You’ll need your Windows CD to install these files. The one with Thai, Korean, etc. is about 10 MB, I think; the one for Japanese and Chinese is about 220 MB or something.
In Windows Vista, all this has changed somewhat, and I’m not sure how accurate these descriptions will be in Vista.