Wow, at first I thought it was 38Mbps and I was thinking "That's not that bad" until I noticed it was KB. I had this exact problem in Windows 7 a few months back when attempting to tweak my dma2100's network performance. When switching one of my adapters to 100Mb full duplex when the rest were 1000Mb full duplex I had speeds very similar to yours.
If you haven't already check the advanced properties of you LAN adapters for all of your PCs on the network.
1. Type "ncpa.cpl" in the start menu to open network connections.
2. Right click on your LAN adapter and choose properties.
3. Click configure, and then the advanced tab.
4. Choose the "Speed & Duplex" option and make sure it's set to 1.0 Gbps Full Duplex on all of your PCs.
If you're using a wireless bridge to join parts of your network you of course won't get 1.0 gbps, but only the speed of the wireless connection. It should still be faster than 38k though