There are several things to check in Win7 that could be causing this...

-Windows Firewall (should be off for the local network)
-Sharing settings (found under Network and Sharing center)
-Homegroup settings for sharing (if one was setup)
-Are you using a username/password to connect to the printer? If so, was this account created on the Win7 box just as it was on the XP box?

Cheers!