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I am running WinXP as a guest on Win7 notebook host, connected to WAN via VDSL. Therefore MTU is 1492, MSS is 1452, TCP Window (RWIN) is set to 64240 by default. Network is configured to bridge-mode.

According to SpeedGuide.net TCP/IP Analyzer this limits the connection speed to 2570 kbps (321 KBytes/s) @ 200ms.
I double-checked this on speedtest.net which reveals higher values but these were not that high as measured directly on the host.

I tried to improve the guest networks settings with TCP optimizier, but there is only one adapter listed (AMD-PCNet Family Ethernet adapter).

What's the relationship of host and guest TCP/IP settings? My assumption is that's not possible to run the network adapter with two different settings. But what if the settings of the host do not match the settings of the guest.
What's their impact when running the adapter in bridge-mode?
Is there any chance to improve the guest network speed?

Hoss wrote:What's the relationship of host and guest TCP/IP settings?

Assuming you are talking about bridged networking, there is no relationship. The two PCs are building TCP packets independantly, using the NIC independantly, and their own individual rules apply to their own packets.

I just did a speedtest using speedtest.net using my Xubuntu Host and Windows 7 Guest. Results are surprising. The Guest is about 10% faster with down and upload! Browser used: Operan 11.50. Might be flash, but other than that, a 10% difference is quite something. If you have speed issues, then you might want to take a look at what is running in the Guest as well, and the system load. If your CPU is overloaded, then it can give you false speed readings, because the CPU does have to handle the end traffic.