Vista - Network access error

i have 3 computers networked together using a workgroup setup with a router, and all computers are hard wired into the router (no wireless connections, wireless disabled in router). my computer has vista x64 ultimate installed, the other two are windows xp (one is pro and the other is home) initially everything was working great, but now i can not access the windows xp pro computer thru the network. it lists it in network on the vista x64 box , but when i try to access it, it says windows cannot access \\mainhome\user 0x8007005 access is denied. the windows xp pro box does not have any passwords for access set (logon pass, etc.). i have let it run diagnose on the vista x64 box and it says that windows did not find any problems with this computers network connection. i have checked all settings, made sure that all names were spelled correctly, but i still can't access it. the funny thing is that i can access the vista x64 computer thru the network using the xp pro machine without issue. the xp home machine can access the x64 box, and i can access it from the vista x64 machine but it also cannot access the xp pro box. any help, or a redirect to where i can find info to resolve this issue will be greatly appreciated. thank you in advance