use below code, it may help for you (http://weblogs.asp.net/gurusarkar/archive/2008/09/29/setting-authorization-rules-for-a-particular-page-or-folder-in-web-config.aspx):
<configuration> <system.web>
<authentication mode="Forms"/>
<authorization> <deny users="?"/> //this will restrict anonymous user access
</authorization> </system.web>
<location path="register.aspx"> **//path here is path to your register.aspx page e.g. it could be ~/publicpages/register.aspx**
<system.web>
<authorization>
<allow users="*"/> // this will allow access to everyone to register.aspx
</authorization>
</system.web> </location> </configuration>