I've got exactly the same problem. Actually I've got three machines with Windows 7 and one is not accessible by the others although it can be seen in the homegroup but the icons of shared folders are faded and inaccessible. Although I can access shared folders of the other two from that machine which is a laptop as well by the way. The homegroup is created in x64 Windows 7 desktop. This desktop has got a Agnitum outpost firewall which caused an issue for the laptop to connect to the homegroup. I managed to connect with the firewall disabled but the no access issue is still there even if I keep the firewall disabled. All three are connected to a D-link 2540 router.
System Manufacturer/Model Number Custom OS Windows 7 64 bit CPU AMD Athlon II x3 (unlocked to a Phenom x4) Motherboard ASUS M4A77D Memory 4gb Patriot Viper Graphics Card Sapphire Radeon HD 5770 (1gb) Sound Card On board with cheap 5.1 speakers, lol Monitor(s) Displays ASUS VH198 21" Screen Resolution 1440 x 900
Keyboard Logitech wireless Mouse Logitech wireless PSU Thermaltake 500w Case Gigabyte Triton 180 Cooling Thermaltake TR2-R1 for CPU, 120mm stock case fans Hard Drives 250 gb Western Digital
500 gb Samsung