Looking for customizable 'portal' software

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I'm looking for an ASP.NET package that runs forums, downloads, user
inbox, etc... but it not a entirely self-contained system. I need to be
able to build this into an existing marketing based ebsite (i.e. with
lots of static content pages)

that's a very simply brief, but does anyone have any ideas that I can
check out?

Most solutions I've found (DNN, MS Sharepoint) are too specific or too
insular. I need to be able to customize the system and integrate it's
features (such as user login) into other pages, like my site's home

Quite honestly, I'm expecting to have to write this whole thing myself
from scratch, as I did for my current site (www.questionmark.com), but
I'm old-school and that site was written with ASP and I really want to
go to .NET, but without the steep learning curve. That said, it'll be a
challenge and that's not a bad thing.

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