As CommonTater explained, the Windows 7 Homegroup feature has no value to downlevel clients like Windows XP, so if the Win7 Machine is in a HomeGroup then choose leave HomeGroup from Network and Sharing Center.
It would seem that the netbios cache table contains some invalid entries.
Open a command prompt, Start Orb, All Programs, Accessories, Command Prompt right click choose run as administrator.
type nbtstat -R this should purge existing information in the netbios name cache
then use the ping utility to ping the XP machine, first by IP Address, and then by Name
then examine the netbios name cache by typing nbtstat -c you should see the XP machine name associated with the correct IP address. I suppose it's possible that if the XP machine is multi-homed (multiple network interface cards) and both cards have Netbios over TCP/IP enabled on the same subnet, then you may get multiple entries in the netbios name cache table.
You may want to address this by disabling the one you are not using. Machine names (host names) need to be unique.
Usually the error you are getting is a product of some type of third party software running on the XP machine that is misinterpreting the attempts by the Windows 7 machine to access shares on the XP machine as a network intrusion. So if you have any additional firewall products or internet security suites running on the XP machine, make sure that you have included the Windows 7 machine as a trusted network node. I would usually recommended uninstalling any such products from the XP machine to make future diagnostic steps less problematic, you can always reinstall the software after the problem is resolved.