Is it possible that you have added a language to your Regional Settings?
To check that (in XP), go to Control Panel / Regional and Language Options / Languages. Click the Details button. You should see a list of your installed keyboards.
Normally, you would expect to see just one item in this list, this being the keyboard for your language and/or region (probably "English (Australia)".) But if there is more than one entry there, that could explain the problem. If so, delete the one you don't want.
I used to have both a UK and USA English keyboard installed, with USA being the default. Like you, my applications would randomly switch to the UK layout every so often, and I had to close and restart the program to get back to normal. In my case, the issue only affected four or five characters, but with other languages (or with other layouts, such as Dvorak), it could be much worse.
I know this is a bit of a long shot, but it's something that would be worth checking.