Two possibilities come to mind. As decepticon mentioned either hard drive problem or bad NICs. Easy checks to eliminate the NIC problems are two-fold:
1) Uninstall/re-install NIC's on both systems via Device Manager
2) open an administrative cmd prompt (right-click, runas administrator) and enter "netsh int ip reset c:\tcp.log" (without quotes).
The first part checks for corrupt drivers, the second part will reset your TCP/IP stack in Windows. You should perform these steps on both systems to have your bases covered.
If you're still having a problem then you may have a drive failing on you. The first indicator of a failing drive is slow performance overall (takes forever to startup, programs slow to launch, menus sluggish, etc). Give the NIC rehab above a try and if you're still having a problem post back here and I'll go into the hard drive remediation steps I go through in these situations.
BONUS: Temporarily disable any AntiVirus on both systems while doing the transfer. Some AV scanners hinder file transfer performance, especially when there are a lot of small files involved.