Edit: problem now fixed...
Thanks, Storm. I followed your instructions, and everything worked perfectly. I was able to map a network drive in my virtual WinXP right off the bat.
Then I rebooted the virtual machine after installing a program. Apparently that was the stupidest thing I've done in at least a week. Upon rebooting, I tried to access the network drives, and the virtual WinXP asked me for my username and password. I entered the same things as last time ("ROB-DESKTOP\rob" for the User Name and the password set in "sudo smbpasswd -L -a rob" for the password). But this time, pressing enter just redraws the password window, ad infinitum.
I know my smb.conf is proper since all three of my desired drives mounted the first time. Does anyone have any idea how I managed to foul up a perfectly functional setup?
Solution: I replaced "ROB-DESKTOP" with "ROB_DESKTOP" (hyphens became underscores) when mapping the network drive. I'm pretty darn sure the first time it worked, it was spelled with a hyphen, just like the hostname is shown in the Konsole. Can anyone explain that?