ARUMBU  can be downloaded from their home page http://kanithamil.com/arumbu/ArumbuSetup.exe for free.
Best things first.

• ARUMBU modifies your regional settings to Indian language to Tamil without Windows XP CD, which means you need not struggle in enabling 'Unicode'.
• ARUMBU  is a 'light' software that weighs around just 860KB.
• ARUMBU enables you to "Hot typing" which means you can write Tamil in any website, mails like (Yahoo!, Hotmail, GMail, MS Outlook), Chat applications (Yahoo Messenger, Google Talk) and wherever the text areas are there.
• ARUMBU installation come with FREE 10 UNICODE FONTS

Installing ARUMBU

1. Download the software and install it as usual.
2. The software will prompt for a system restart to modify the regional settings.
3. The default option is that NHM Writer will be starting along with the Windows OS.

Writing using ARUMBU.

You can find the ARUMBU Writer in a Blue  Tamil icon   அ at your status bar. Right clicking it will give further settings options, Keyboard preview and a left click will enable the keyboard layout for typing. You can enable it by using Keyboard shortcuts also (Alt+0, Alt+1, Alt+2, Alt+3, Alt+4) in this case.

General Typing using ARUMBU

• Alt+0 - Turns off the Tamil typing
• Alt+1 - Enable Tamil 99 Unicode
• Alt+2 - Enables Tamil Phonetic Unicode
• Alt+3 - Enables Tamil Old Typewriter Unicode
• Alt+4 - Enables Tamil Bhamani Unicode


I don't know the keyboard layout for typing in Alt+1, Alt+3, Alt+4. Tamil Phonetic Unicode is suited for transliteration, so that you can type the words in English font itself. So my experience / this tutorial will be limited to option Alt+2 only.

உயிர் எழுத்துக்கள்:- 
அ - a, ஆ - A, இ - i, ஈ - I, உ - u, ஊ - U, எ - e, ஏ - E, ஐ - ai, ஒ - o, ஓ - O, ஔ - au  
 
மெய் எழுத்துக்கள்:- 
க் - k, ங் - ng, ச் - c, ஞ் - gn, ட் - t, ன் - n, ண் - N, த் - th, ந் - w, ப் - p, ம் - m, ய் - y, ர் - r, ற் - R, ல் - l, வ் - v, ஷ் - sh, ஸ் - S, ஹ் - h  
 
உயிர் மெய் எழுத்துக்கள் (e.g 'அ'கர வரிசை = மெய் எழுத்து கீ + a) 
க - ka, ங - nga, ச - ca, ஞ - gna, ட - ta, ன - na, ண - Na, த - tha, ந - wa, ப - pa, ம - ma, ய - ya, ர - ra, ற - Ra, ல - la, வ - va, ஷ - sha, ஸ - Sa, ஹ - ha