Before Avast, norton 2003 was running on the machine which I uninstalled and runned the norton 2003 removal tool.
After restart I installed Avast (for the first time).
I read some messages on the TMT forum from people who have the same problem, but the only solution was to de-install Avast. Which i've done now and TMT 3 is working again.
Somehow TMT 3 hits Avast in a way it triggers a reboot.
I was very amazed that TMT 3 and Avast was working for you. Are you on Windows XP, Vista or Windows 7 32/64 ?