I think many problems are caused by the confusion Microsoft has introduced into the home network environment by supporting two very different methods of handling sharing and permissions. Because so many acquaintances know me as a "computer guy" (I'm not, really, but that's how they see me -- perhaps because we have 8 systems on a network at the house.) and because I'm a pushover I have set up at least a couple dozen WinXP home networks over the past couple of years. The most serious troubles I've faced in establishing file sharing have come from people who are used to the regular domain or workgroup methods of setting up file sharing having worked on those networks before I got there.
Apparently, MS figured that life would be easier for home networkers if they provided this alternative means (using the Guest account as a template for accomplishing the sharing). Perhaps, if MS had been more explicit / precise and had provided some warnings about not trying to mix methodologies they could have avoided a lot of the trouble so many of us have had with this. The problem is, I think, that they just assume that anyone who uses these tools is totally clueless about networking. You almost are better off if that's the case.
It's made all that much worse by the fact that, even if you do set up a small home network "properly" using simple file sharing it seems, many times, to take a number of minutes for all of the WinXP boxes to pull their heads out of their butts and see each other. I've seen people take this initial recalcitrance as a failure and start fiddling with configurations. That, of course, almost always leads to a real failure.
I don't know what to tell you about your situation. As you well know there are a LOT of little issues which can defeat you in a case like this. Just one little setting out of kilter results in no soap. Trying to rectify a botched setup by changing one thing at a time and testing can be a very painful experience. My best advice, as I mentioned before, is to attempt to take these boxes back to non-networked / non-sharing configuration. That's not always really easy to do. I wish to dickens that MS would provide a "reset-to-square-one" button for the networking / file sharing configuration. Once you get there, go with simple file sharing all the way and follow a step-by-step like the one at ezlan.net.
If you can't get sharing over TCP/IP to work you could try that guy's instructions for using NetBEUI. It really does work marvelously well, and I set that up for the totally clueless whom I'm afraid will get themselves in serious trouble if I set up sharing over a routable protocol for them. It's really a viable security precaution for sharing on a small network -- if you don't have to share with non-Windows boxes. You're obviously not clueless, but you are frustrated. NetBEUI would beat frustration.
Of course, if I couldn't get sharing to work over TCP/IP it would drive me nuts -- just because. I'd take the danged boxes down to bare metal and start over, if I had to, in order to get them to "work right". But I've never really had to go that far. I think that, if you can make sure that permissions and security settings are set back to the default and start from there with the step-by-step it should work -- unless some part of one or more of the systems got munged along the way.
Sorry to be so long-winded while actually saying so little. I'll keep a pair of fingers crossed for you.