Assuming you have a wireless N card on your PC, and assuming you've set up a dual band network with your Time Capsule, you should be able to utilize the 802.11n 5GHz band. As I am not familiar with the new TC (I have last gen TC), I'm not sure how you'd set it up for this, but I do know how to make the PC recognize the 5GHz band.
On my Sony VAIO laptop, I had to click the LAN icon (green icon in status bar of Windows) and select the 5GHz option. Prior to doing this, it would not detect the network as the wireless card in the PC was not set to look for it. That may be the case in your situation which would explain why it only detects the 2.4GHz 802.11g network.
However, if your PC's wireless card does not support 802.11n, you'll need to purchase a new card that does.