>I am developing a app for many customs. Each time a make a little (or great) modification of it, must i send to all of them the entire new exe file? or ther's a way to update the existing one ?. Is very important cause the exe file could be higher than 3 Mbytes. Even if i use E-Mail to distribute it time is gold for me and for the others.

You might consider using patch file generators such as .RTPatch (practically
industry standard software update system used by MS, Borland, Novell among many
others). The way it works is by comparing the newer version of the program
against the older one and extracting detected changes into a separate patch file.
You would send that patch file to your clients and it will auto-update all the
altered files. We've been using it for a year now and I can only say good things
about it. It's extremely configurable (you can include entire directory trees to
be scanned for changed files and it will deliver all the updates for you at the
client's site) The patch files are usually around 30K - the biggest one I ever got was about 90k.
One caveat though: it cannot be used with Fox-'encrypted' files.

You can quickly download an evaluation version from http://www.ptsdirect.co.uk/static/rtpatch.html

Their homepage is at http://www.pocketsoft.com