This was achieved (I think) partially by reinstalling the TCP/IP stack, something that while it's possible to do under Windows 2000, is not supported at all by Microsoft and doesn't quite work. I had reasons for doing so at the time (had broken the firewall with some network card shenigans), and did so knowing that it was a possibly stupid thing to do. I think at the time it came under the heading of being a lot quicker than reinstalling everything. I'm not sure what the other things were that caused this, but knowing what I do to the system it could have been anything.
I'm not sure what the Linux analogy would be, but it probably involves breaking some part of the kernel in a way that while you could repair it, it would be a lot simpler to reinstall and so clean out the various bits of crud that have accumulated since you last installed Linux. Config files from non-existant programs, libraries that nothing uses anymore, bits left from installing programs, temp files from programs that don't clean up after themselves and don't use /tmp - I'm sure that Linux software is just as badly behaved as Windows stuff. It's probably possible to unpick this mess I made and get BITS and IIS working again, but I couldn't be bothered to do so.
Plus, my Norton subscription is about to expire. This is why I'm doing the reinstal now, as opposed to going "well, I don't really need those bits and I can't be bothered to go through the hassle of reinstalling everything".
I do this about once a year, usually because of some spectacular failure. Last time it was because the system disk decided to randomly corrupt everything and crash hard on startup, the time before was because a system crash (blue screen of death) conspired to completely hose the Norton install in such a way that after reinstalling Norton it wouldn't let me activate it. Norton hides stuff everywhere, and the instructions on their site don't reveal all the files. This might be the first time that I've done a reinstall by choice, rather than have a major failure force it on me to restore acceptable functionality.
This isn't typical Windows usage by the way - I know several people who are still running the original install of things as old as Windows 98, and are having no problems with it at all. These people usually run Office, a Photoshop clone, maybe a few older/less common games, some light web browsing and email. They don't use it as a development enviroment and try to do strange things to the hardware/software setup to get round uncommon problems. But you only hear the people who have problems, and do stupid stuff.