Is it possible to access to the registry from a web application?
If you mean the client's, then no. (It might be with some ActiveX controls, but I am not sure on that)
Why do you need to be accessing the registry from a webapp anyway?
The registry, as in the Windows registry? It might be possible with Internet Explorer 6, but I don't think so. There's no possible (good) reason to access the registry on a website, you just don't want that!
You can't access the client's registry no, you could access the registry on the server but that would be a fairly iffy thing todo in a web app.
It is completely impossible to access the end-users registry, unless you use an IE-only ActiveX control. (and there is (almost) no better way to make your users hate you than to use an ActiveX control)
If you're trying to check your end-users' home pages, you should give up; it's completely impossible.
After reading you comment to @SLaks I'm inclined to point out that there are server tools that are there to do exactly that.
In fact I think Windows Server editions come with these features out of the box.