Back to the logins, password thing, I personally used the Windows Client Login (Client for Microsoft Networks) on all machines and required a username/password on all machines (and made sure they matched) back when I had some Windows 98 machines. I only have Windows 2000 and XP, now so that's from memory. Since you are able to get everywhere when the switch is out of the picture, I don't think this is the source of the problem.
On the Windows XP Laptop, bring up your Network Connections window and look at the properties for the Local Area Connection. If you open the properties for the Internet Protocol (TCP/IP) and click on the Advanced button then the WINS tab, is "Enable NetBIOS over TCP/IP" the radio button that is checked? I would try that as opposed to using the Default setting.
I also just noticed that on this machine, I've Installed the additional protocols "NWLink NetBIOS" and "NWLink IPX/SPX/NetBIOS Compatible Network Protocol" I think I did that specifically to allow for running Diablo II network games, but maybe it's had a side benefit. You might try installing those from the Local Area Connection Properties dialog. (You can always Uninstall them later.) This machine is probably running the oldest XP configuration I have, and it was running in a mixed XP/W2K/98 environment for quite a while. It was always sharing printers and folders (and still is).