the following will let you kill a stubborn service.

1. Go to the command-prompt and query the service (e.g. the SMTP service) by using sc:
   
   sc queryex SMTPSvc
   
2. This will give you the following information:
   
   SERVICE_NAME: SMTPSvc
           TYPE               : 20  WIN32_SHARE_PROCESS
           STATE              : 4  RUNNING
                                   (STOPPABLE, PAUSABLE, ACCEPTS_SHUTDOWN)
           WIN32_EXIT_CODE    : 0  (0x0)
           SERVICE_EXIT_CODE  : 0  (0x0)
           CHECKPOINT         : 0x0
           WAIT_HINT          : 0x0
           PID                : 388
           FLAGS              :
   
   or something like this (the "state" will mention stopping).
   
3. Over here you can find the process identifier (PID), so it's pretty easy to kill the associated process either by using the task manager or by using taskkill:
   
   taskkill /PID 388 /F
   
   where the /F flag is needed to force the process kill (first try without the flag).

Please be careful when you do this; it's useful for emergencies but you shouldn't use it on a regular basis (use it as a last chance to solve the problem or to avoid the need of a reboot in an exceptional situation). It can even be used to stop a service that has the "NOT-STOPPABLE" and/or "IGNORES_SHUTDOWN" flag set (e.g. Terminal Services on a Windows Server 2003 is non-stoppable), at least when it's not hosted in the system process. You can query all this information by means of the sc command.

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/sc.mspx