Slow network speed between Laptop and PC

Ever since I built my new PC last year, I've had terrible transfer speeds between it and my laptop.

Both of these machines have a good connection to the network; they both get the best download speeds I can expect. (The laptop uses wi-fi and the PC is wired.) But when I try to transfer files between them over the network, I'm lucky to get 15 k per second.

But it is just between these two computers. I have my old Windows XP computer on the network, and I can transfer files across it all full speed. With large files it actually becomes faster to turn on my old PC, transfer files to it, and then copy the files over from it, rather than directly transfer anything between the new PC and the laptop. So this isn't a problem with the actual network, but something in the network settings on these computers.

The laptop runs Windows 7 and the new PC runs Windows 7 64-bit.

Typically I am on the laptop dropping files to/from the PC, because I have my D drive shared on the PC.

I looked at my share settings on my PC, but I didn't see anything that sounded to me like it could be causing this. Networking isn't my forte.

Have you tried using Ethernet to Router to Ethernet transfer to see if Wi-Fi is the problem?

As I said, perhaps not clearly enough, I have no problems transferring files with my old Windows XP computer.

I can copy files from my new PC to my old one, and then from my old PC to my laptop with no problems at full speed. And the other direction as well.
As such, I know the problem is not with the wi-fi connection, the problem is my windows 7 computers talking to each other.