my client has this error
Fatal error: Session variable IsAdminLoggedIn is not globally defined
when using oscommerce. Anyone know how to solve it?

what version of PHP is on the server?

PHP 4.3.7 will that be any prob or do I need to upgrade to 5?

Would not suggest 5 but 4.3.7 is old. 4.3.9 is the latest stable I believe.

Ive had oscommerce working with 4.3.8
Try searching for the error message on google and on the oscommerce forum
http://forums.oscommerce.com/