I had the exact same problem except that my W7 PC couldn't use the network printer attached to my XP PC. The fix was wholly and entirely not intuitive - and I wish I could remember what it is. I vaguely remember something like, instead of telling the W7 PC you want to add a network printer, you tell it you want to add a local printer and then tell it the printer's location as \hostname\printername - something like that. I found it by extensively Googling. I'm sorry I can't be of more help.
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