[TYPO3] typo3 4.0.1 install

[Rocky]

Hey Group,

Right now we have a static html website up and running under public_html 
folder which could be accessed through www.domain.com. I'm thinking of 
installing typo3 4.0.1 in public_html folder while keep the current site 
untouched. After extract the typo3 source and dummy in the directory I 
moved the files under dummy-4.0.1 into public_html. The problem is that 
I got a 404 error when try to finish the installation by 
www.domain.com/index.php.Well, I extract the dummy package again and did 
not move the files under dummy-4.0.1 directory and access it through 
www.domain.com/dummy-4.0.1 I can get the installation started. Can any 
of you tell me how to solve the problem please?


Thanks a lot in advance!
Rocky