Vista - Vista and XP Networking Woes

I have a private home network consisting of two computers, one running Windows XP home and the other Windows Vista x64 home, connected via a router. The XP computer is running SP3 and the Vista computer SP2. Both are members of the same workgroup and have file sharing enabled. I am able access the shared files from the Vista computer just fine, but when I use the Vista computer to try to access the shared XP files, the computer and files are conspicuously absent. I am able to see the XP computer by mapping the network however. I also cannot ping the XP computer from the Vista computer. Both are using the same antivirus and firewall software. Disabling the firewall does not improve the situation any. I am at loss for what more can done to rectify this one-way communication higway. Please help.