Most services on Windows NT/2000 Server can be started and stopped either via the Services control panel or by issuing the DOS command "net [start|stop] servicename". This is NOT the case for the Oracle service.
Using "net start/stop" with the Oracle service is a bad habit to get into. Depending upon your registry parameters, stopping and starting the service may not actually bounce the database. It can bounce the service only; the database only; or both the service and the database.
Oracle recommends that you use their Oracle-supplied utility, oradim.exe, to start & stop the service. (OraDim is also used to set up and configure the service & the above-mentioned registry parameters; more about this here.) Unlike net start & net stop, oradim lets you supply the shutdown type (normal, immediate, or abort) and shutdown mode (instance, service, or both.
C:\ORACLE\ORA81>ORADIM -SHUTDOWN -SID-USRPWD - SHUTTYPE -SHUTMODE
NB: oradim.exe has different names depending on the version of Oracle. For example, ORADIM72.EXE.