Big transfer locks up computer

Hello all,

I recently set up two new Windows 7 computers (laptop and desktop) -- and the following problem has emerged. Previously, I had a Windows 7 laptop and a Windows XP desktop, but I'm still using the same router, so I assume that's not the issue.

Here's the problem:

I use my desktop as a server to my laptop. I will transfer data from my laptop (maybe on a USB or a folder) to my desktop over the network (the laptop connects to the router via WiFi, the desktop is connected via ethernet cable).

While transferring a large file (say, upwards of 1GB), if I go to open a folder on that desktop computer, it takes an agonizingly long time. The entire operation of the desktop computer is nearly halted. I can still work on the laptop, while the transfer is occurring, but the desktop is crippled.

Any ideas as to what's causing this? As I said, I did swap out the two computers, but the router is the same -- and, before, when I did this (which is about once a week), I did not have this issue. The transfer worked in the background and didn't cause the desktop computer to grind to a halt.

Thanks!

Chris