Given the additional information, look closely and firewalls and antivirus. You could disable these temporarily and see if that helps. Also (I know this to be an obvious point), have you tried pinging the share? Again, given your feedback, try:
NET USE DRIVE: \\wpipaddress\share and answer the prompts
There may be an issue affect names.
Just by the way, I follow myself the advice we have been discussing and I connect my Vista Business 64-bit laptop to my XP Pro (SP3) Desktop all the time.
... Thinkpads_User