As suggested my Microsoft we would be deploying our custom pages in ISV folder within CRM’s virtual directory i.e. CRMWEB.
Now within our site map or isv.config if we are referring to the page using relative url
i.e. /ISV/HelloWorld.aspx, what CRM would do is append Organization Name to the url.
Now as long it is an aspx page, things would be fine. However if it is an html page we would get “Page not found error”. And for html page we need to use Absolute url, for them to work fine!
Check out this url as well
Using Microsoft Dynamics CRM URLs
And this thread as well