Multisite headache!!! Sub-directories not working!

Hi all. I hava a multisite installation with subdirectories enabled. Main site works fine. Nothing weird. But when I create a new site, It doesn’t work at all. I can’t access to sub-site admin also.

My sites looks like: http://www.mainsite.com and http://www.mainsite.com/subsite

I get this message:

“The page cannot be found
The page you are looking for might have been removed, had its name changed, or is temporarily unavailable.

Please try the following:

Make sure that the Web site address displayed in the address bar of your browser is spelled and formatted correctly.
If you reached this page by clicking a link, contact the Web site administrator to alert them that the link is incorrectly formatted.
Click the Back button to try another link.

HTTP Error 404 – File or directory not found.
Internet Information Services (IIS)

Technical Information (for support personnel)

Go to Microsoft Product Support Services and perform a title search for the words HTTP and 404.
Open IIS Help, which is accessible in IIS Manager (inetmgr), and search for topics titled Web Site Setup, Common Administrative Tasks, and About Custom Error Messages.”

To be sure that Is not a server problem, I made a local installation running XAMPP. Main site works fine but subsite doesn’t. Subtsite urls look ok. I can access to frontend subsite (get 403 error with css files) but not to admin subsite (I get 403 error too).

Anyone have a clue about what’s going on?

Thanks in advance.

ps: sorry for my english.