Gigabit network is slow

[ericus]

Hi all.
I have a problem with my gigabit-LAN. The setup looks like this:
Internet - ClearOS Firewall - Gigabit-switch - clients.

Computer #1 (desktop):
3.4 GHz quadcore, 4GB RAM, onboard LAN (gigabit).
Primary OS: Ubuntu 12.04
Secondary OS: Windows 7

Computer #2 (Windows 2008r2 Server):
3.5 GHz quadcore, 16GB RAM, onboard LAN (gigabit).

The problem is that I can not achive full speed with Ubuntu > Win Server. Maximum speed that I've seen while transfering files is ~25-26MB/s, while Win7 > Win server gives me around 90-110MB/s. So, there's no hardware problem.

From Win Server > Ubuntu desktop the speed is about ~35MB/s.

I have tried with another NIC from Intel (PCI-E gigabit), and got the same poor performance. So what could be the problem here? I've installed the latest drivers for my onboard NIC, without any changes...

Some outputs:
ethtool eth0:

Settings for eth0:
	Supported ports: [ TP ]
	Supported link modes:   10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Supported pause frame use: No
	Supports auto-negotiation: Yes
	Advertised link modes:  10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Advertised pause frame use: Symmetric Receive-only
	Advertised auto-negotiation: Yes
	Speed: 1000Mb/s
	Duplex: Full
	Port: Twisted Pair
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	MDI-X: Unknown
	Supports Wake-on: pumbg
	Wake-on: g
	Current message level: 0x00000033 (51)
			       drv probe ifdown ifup
	Link detected: yes

sudo lcpci -v:

05:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 06)
	Subsystem: ASUSTeK Computer Inc. P8P67 and other motherboards
	Flags: bus master, fast devsel, latency 0, IRQ 49
	I/O ports at e800 [size=256]
	Memory at fafff000 (64-bit, prefetchable) [size=4K]
	Memory at faff8000 (64-bit, prefetchable) [size=16K]
	Capabilities: [40] Power Management version 3
	Capabilities: [50] MSI: Enable+ Count=1/1 Maskable- 64bit+
	Capabilities: [70] Express Endpoint, MSI 01
	Capabilities: [b0] MSI-X: Enable- Count=4 Masked-
	Capabilities: [d0] Vital Product Data
	Capabilities: [100] Advanced Error Reporting
	Capabilities: [140] Virtual Channel
	Capabilities: [160] Device Serial Number *************
	Kernel driver in use: r8168
	Kernel modules: r8168

What could be the problem?