Email confirmation delivery failure for new accounts

I just installed Drupal 5.7 in XAMPP 1.6.4 on Windows XP. The intallation went fine, but after I created the admin user this message appeared:

warning: mail() [function.mail]: "sendmail_from" not set in php.ini or custom "From:" header missing in C:\xampp\htdocs\drupal\includes\common.inc on line 1991.

So the email confirmation wasn't delivered.
I went to the "php.ini" file, erased the ";" from "sendmail_from" and restarted apache, registered another account and the same problem.
Any solution?

Also, I wonder if this is a common problem in a shared hosting account.

Thank you for your colaboration,

Cristian