Network overhead cost of CFP on gigabit ?

I had noticed for last few weeks a slow down in a backup task across my network.
I only just remembered I had CFP installed yesterday so did some testing.
I only have the Firewall active or installed [not the other bit forgot exact name sorry]
Both machines are running windows xp sp2 - patched upto sometime last week.
They both have latest drivers for the networks and are both stable reliable machines.
Between 2 3ghz core 2 duo boxes with P35 chipsets if CFP is active on my main
computer the maximum bandwidth throughput is around 200mbits.
If I disable CFP it is still 200mbits
If I uninstall CFP it is 800mbits
The measured the bandwidth availability using iperf to exclude any disk performance issues.

Is this considered normal ?