Along with a change to TCP offloading on our server, a staff member on a Vista machine started encountering a problem when logging in; her computer would hang with "Windows is waiting for the user profile service" on the screen. Checking the service status from a remote computer showed this service as "Starting..."
Although I really wanted to remotely debug this issue (grab a stack trace with a debugger attached), the user was anxious to get back to work which means I had to work quickly.
I stumbled upon a feature named TCP auto-tuning or TCP window scaling. It turns out, this feature also ties into TCP offloading, which we just disabled on our server due to a Broadcom issue.
You can turn off auto tuning from the command prompt by running
netsh interface tcp set global autotuninglevel=disable
Sure enough, after running this (from safe mode), the user was able to login again.