Windows 2003 SP1 - Slow Network

Any reason why intalling SP1 would cause my file transfer rate to drop in
half. Without SP1 I get 60Mb/sec with SP1 I get 26Mb/sec. I disabled DEP
after installing SP1 but no change in the outcome. NIC drivers are up to date
and using Robocopy to copy 4 X 1 GB files in my test. I manually calculated
the rates that Robocopy was showing and they are accurate. Any ideas or
suggestions would be greatly appreciated.

Thanks
CT