windows wants network credentials pw to access mac

i have 2 pc's -- one winxp and one win 10. whenever i try to access the mac from either one of the pc's for file sharing, a "windows security" dialog box opens wanting a UN and PW. i was advised to set all (mac and both pc's) with the same login pw -- i did but to no avail. the pc's still want the network pw. i can however access the pc's from the mac without a pw. what is so baffling is that i have never, to my recollection, set up a pw on either of the pc's.
the mac has yosemite 10.10.2 (if that's relevant). don't know where to go from here-- any help greatly app. tks in adv
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