1)
Normally the Realtek 8169S 1GBit Nics are running 100% stable
with newest Realtek Reference Drivers.
I use these NIC since approx. 2 years now without having any problems.
If you want to be sure, that your NIC is not the problem
in this case try to install newest drivers
you can download here:
http://www.realtek.com.tw/downloads/...&Software=True
after this update install the "Windows diagnostic utility" too.
After a clean reboot check your Diag Tool.
Now you can see if you got any Send/Receive errors
over some time or your network-cable is weak.
Additionally i ALLWAYS recommand to set the NIC-Link-Speed
explicit to the Router/Switch's connection-Speed.
For example: If your router has 100MBit LAN Full Duplex, set it so.
You can do this via the Diagnostic-Utility too (or inside the driver).
Mostly the standard "Autosense"-Mode is working fine,
but not with all kind of network devices
(some Cisco-Switches and others have known issues...).
In such cases you can get (massive) problems on your LAN.
(like runts ect.) which can cause crc-errors above average.
2)
Additionally CHECKDISK your Harddrives.
Maybe your HDD/HDD-Controller is not working correctly.
Especially if you are using external HDDs over USB/Firewire
and mix different devices together on same bus,
it can cause CRC errors too.
It seems so that some controller-chips
are not compatible with others.
That's why I use only 1 drive exclusiv on USB/Firewire-controllers.
3) as mr_F said before, it can be your RAM too.
You can found several RAM checking tools in internet.
Google for it and check your RAM, if it works stable.
Wrong BIOS-configuration (RAM-timings, Overclocking)
can often cause such CRC-errors too.
Try to reload the standard-bios-settings for it.
I hope this will help you out... ;-)
I didn't know where else to look for help.. 
