If you have problems with the wrong link in your email when either a user signs up to your site or someone replied to a post you are watching then this should fix it..

Steps :
Log into your phpnuke admin panel and click on Preferences. Check the site url and make sure it points to the folder that phpnuke is installed without any slashes like the following.

http://www.yoursite.com/nuke

Also go to your fourms configuration and make sure you have the same url as above as your "Domain name"

That's all!. More help in the forums if you need it.