I've covered something like this before here but can't remember where! The problem is nearly always with the "receiving" PC.
In the Win2K default set-up, "Guest Accounts" are disabled.
On the Win2K machine, go to Control Panel/Administrative Tools/Computer Management/System Tools/Local Users & Groups/Users. In the right-hand window, right click Guest and left click "Properties".
There's a box there for you to enable/disable Guest.
Fix that and it should let your XP machine in.