I just got this working with my PSP v2.0.
I have Win XP Pro and it coes with IIS, so does Win 2000 and Win 2003.
Simply setup a virtual directory in IIS with directory browsing enabled and enable the HTTP port in your firewall and presto, instant fileserver.
You can access the fileserver at "http://fileserverIP/" through your PSP's browser.
You get virtually unlimited storage (depends on your hard drive of course) and no limit on file size, also you get quick downloads, I've clocked about 200KB/s on my 108.11g network.
Edit, umm, sorry, maybe this should be in PSP General... mods?