Yes, the check box indicates the status as LMCE sees it, but if unticked you can tick it and it will try to force back on line - usually takes a few seconds to fail if it is going to, so keep hitting refresh.
If you find the tickbox keeps unticking itself, go to a terminal or KDE desktop and try navigating to /home/public/data/audio/Windows Share <name> look for your media and try to play some. Sounds strange, but under some circumstances the media is perfectly visible and playable to KDE desktop, but not in the media grid in LMCE. Incidentally, in those circumstances the media is still findable and playable in LMCE using the Search function rather than browsing the media grid. I have found this in the past related to the smbfs/cifs issue. Sounds like that isn't an issue for you, but worth a try esp as I was getting the CIFS error -11 as well (but not the others you mentioned)
If the autodetect process does not prompt you for username and password for your shares (use the LMCE structure BTW!) then enter them in the web admin - personally I put them in both the server and share objects to be sure!
Try enabling security logging on your XP boxes and look for successful and failed security audit events - these will tell you which account LMCE is trying to use and whether it is the username or password or something else that is causing a failure. Let me know if you need to know how to do this.
Get a known good cat5 cable (seriously, go to the store and buy a cat6 patch lead and move one of the XP boxes next to the core) and connect the core directly to the XP box with no switch - modern NICs shouldn't require you to use a cross over for this. This will eliminate your cabling and switch, which can often cause errors, drops, autoneg probs, etc.
Get a new NIC card that is listed in the wiki as good for LMCE and put that in the core - try that instead in case of driver issues...