I have been working with Win 7 for a year. On my network is another machine running Win XP with a HP Laserjet 2100M attached and I have been sharing the printer successfully until just recently. I renamed the Win XP machine on my network. I had to revise my credentials on Win 7 to properly re-connect to the Win XP machine and can now share files successfully but cannot connect to the printer. My old printer connection fails because it had the old machine name. I deleted the old printer connection and cannot create a new one.
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Can you see the printer share when you run \\xpcomputername from the run box? If you see it, right-click on it and choose connect. It will probably prompt you for a driver, especially if XP is 32 bit.