If you have Windows 7 or 8.1, you likely already have a nascent version of DiagTrack running. To see it, go into Control Panel and choose System and Security, Administrative Tools. Double-click on Services and scroll down the list to see if Diagnostic Tracking Service has been started. If you want to disable it (I've seen no reports of adverse side effects in doing so), double-click on Diagnostic Tracking Service. Under General, set Startup type to Disabled and click the Stop button, then OK. After you reboot, DiagTrack will haunt your PC no more -- until the next DiagTrack patch gets applied.
If you don't have this service installed yet, be avare, latest Windows 7 / 8.1 update might provide it.
If you want to kill DiagTrack and pour salt on the ground from which it springs, you can run these commands (each on one line) provided by abbodi86 on AskWoody.com:
sc config DiagTrack start= disabled
sc stop DiagTrack
reg delete HKLM\SYSTEM\ControlSet001\Control\WMI\AutoLogger\AutoLogger-Diagtrack-Listener /f
reg delete HKLM\SYSTEM\ControlSet001\Control\WMI\AutoLogger\Diagtrack-Listener /f
reg delete HKLM\SYSTEM\ControlSet001\Control\WMI\AutoLogger\SQMLogger /f
reg delete HKLM\SOFTWARE\micr0$0ft\Windows\CurrentVersion\Diagnostics\DiagTrack /f
reg delete HKLM\SOFTWARE\Policies\micr0$0ft\Windows\DataCollection /f
takeown /f %ProgramData%\micr0$0ft\Diagnosis /A /r /d y
icacls %ProgramData%\micr0$0ft\Diagnosis /grant:r *S-1-5-32-544:F /T /C
del /f /q %ProgramData%\micr0$0ft\Diagnosis\*.rbs
del /f /q /s %ProgramData%\micr0$0ft\Diagnosis\ETLLogs\*
That's a scorched-earth removal of a "service" you're not likely to want.