I agree with Gido. On many different machines I've experienced Windows switching my keyboard layouts randomly.
unexist's advice might be helpful for some:
If you don't use both input languages just deinstall the other one.
But I personally use many different keyboard layouts during my daily routine, and don't appreciate them being switched around on me all the time. In English I type in Dvorak. I was forced to install the UK standard QWERTY keyboard by an (otherwise excellent) application I have for learning the Cyrillic keyboard layout (Keyboard Tsar). Even before this, however, I remember my keyboard layout being randomly switched to QWERTY without any intervention on my part. So, for example in a web browser, if I've typed something into a text field and attempt to copy it using CTRL+C, it would send CTRL+R and refresh the page, probably causing me to lose what I had typed! You can imagine how frustrating this might be, especially for a predominantly keyboard-oriented user like myself who multitasks viciously.
I do agree that the default shortcut for switching input languages -- ALT+L-SHIFT could be responsible in some cases -- especially since I sometimes use ALT+SHIFT+TAB to toggle backwards through applications. However, for me, disabling this shortcut has only seemed to work temporarily. Invariably, after disabling it, I will check on the setting a day or two later and find it enabled again! Anyway I've experienced problems with randomly-changing keyboard layouts in Windows XP for such a long time, and I feel sure that much of the time it couldn't possibly be due to my accidentally hitting this shortcut.
I think that a third-party application for better controlling keyboard layouts would be extremely useful. It could have the features to set the keyboard layout system-wide, or to set permanent, automatic keyboard layouts associated with different applications. For example, when I open my Spanish-English dictionary program, it could automatically switch the layout for that application to Spanish international. And it could prevent Windows from capriciously switching layouts, as it seems so fond of doing.
PS, @Adam: Windows manages keyboard layouts per-application. As I touched on in the previous paragraph I'm not always fond of this either, as sometimes I wish to work in a particular language system-wide, and have to change each individual application to that language's layout individually. However, a possible workaround for you -- if you want each application to open with the Korean layout -- is to set the Korean layout as your default input layout. Of course I can understand that this might not be the ideal solution for you, as you may also use English heavily at other times and just end up with the opposite problem if you switched the default to Korean. But I thought I'd just enlighten you about the logic of Windows with regard to this 'feature': in theory, each running application should launch with the system default layout, then remember whichever layout you switch to until it is closed.