Download the Assamese Phonetic Keyboard version 3.2
Check here for the keyboard
layout.
Extract the files and save in your hard disk. Double click the AsmPhon3.msi file and follow the installation instructions.

After finishing the installation of the keyboard layout and if you have Windows XP installed with Indic language support, go to the control panel.
Double click the "Regional and Language Options"

Go to the "Languages" tab.

Click "Details..." button.
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In the setting tab, under 'Installed Services' click the "Add.." button.
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Select ANY indic language like Assamese, Bengali, Oriya, Hindi, Sanskrit, Gujrati, Marathi etc as Input Language.

Select "Assamese Phonetic ver 3.00" as "Keyboard layload /IME"

Close all the open windows by clicking "OK" or "APPLY"
Select the language you selected in step 8 in your Windows "Language Bar".

Open any application - Notepad, MS-Word, MS-Excel or
whatever; select font that has Bengali/Assamese characters in appropriate unicode and start typing Assamese.
Some of such fonts that usually comes with Windows are:
More Unicode Assamese/Bangla fonts are available at
www.ekushey.orgCheck here for the keyboard layout.
You can even name the files in Assamese!!