Like many people, I've had the problem with the "Packet Scheduler Miniport" appearing in the Device Manager with a yellow exclamation mark against it. Even worse, the Lenovo ThinkVantage Toolbox on my laptop picked up on this and put a similar exclamation mark on its icon in the taskbar - a constant reminder that all was not right with the system.
Googling for this turned up various suggestions - none of which worked, although it appears that a lot of people have had problems with this. But I eventually got it sorted out. Here's the deal:
This particular driver is used by the "QoS Packet Scheduler" in the TCP/IP stack, which reserves some bandwidth for any applications that require priority quality of service. The general fix is to open the properties for any network connection and then select this service and uninstall it. This should remove the driver, although it might be necessary to "Scan for hardware changes" in the Device Manager to get the display to update.
Problem is, that wouldn't work for me. However, opening up the properties for this driver revealed a curious fact: the version number shown for the driver was 5.1.2535.0 (same as for many other Microsoft-supplied basic drivers like CD-ROM, etc.) while the version shown for the psched.sys driver file was 5.1.2600.5512.
So, off to regedit to search through the registry for the string "2535". It appears many times, mostly on other drivers, but wherever it appeared as a DriverVersion string for PSCHED entries, I changed it to the file version number of 5.1.2600.5512. Just to be prudent, I then rebooted - I'd wasted so much time on this problem already that a reboot was a small price to pay, then went in to my network connections and uninstalled the "QoS Packet Scheduler", and immediately, in the background, the yellow-asterisked Miniport entry disappeared from the Device Manager.
Reinstalling the "QoS Packet Scheduler" service hasn't caused any problems, either - the Miniport doesn't appear in the regular Device Manager display, but shows up with the other miniports when "View -> Show hidden devices" is selected. This is because those miniports don't normally appear unless they are malfunctioning.
So now you know - the problem is entirely down to the driver version number in the registry not matching the version number in the driver file. Make them agree, and all will be well with the world. I suspect the problem is that XP SP3 updated the driver file, but did not correctly update the registry DriverVersion entries.
I hope this helps others who have had this error niggling at them.