wp-login redirection issue – WP 2.7 Beta 3

Have a blog that refuses to redirect correctly:

http://blog/wp-admin becomes http://blog/wp-login??redirect_to=http%3A%2F%2Fblog

as you can see it is missing off the wp-admin from the end.

server is an IIS6 [due to corporate standards – erk]
php 4.3.9

is this a prob with using PHP 4.3?

Thanks