If I were you, I wouldn't do that.
register_globals is going to be deleted in PHP 6.
And basically it's bad coding. If you use auto global variables, you're not understanding the concept of $_POST and $_GET and other system variables.
If you're trying to install osCommerce for a customer. Tell him to use a different shop.
Register_globals is a security breach.
I advise you to leave it set to default off. Your customer can change register_globals setting on runtime by creating a .htaccess and entering:
php_flag register_globals 1
Again, I advise you to leave register_globals set to 0 or Off.
http://ca.php.net/register_globals - Read. Even PHP admits register_globals is misused for code injections.
Regards,
Johannes Lindenbaum