You have at least three options. I have presented them in order of usage preference.
Method 1 - use sc tool
Open a Command Prompt and enter
sc delete <service-name>
Tool help snippet follows:
DESCRIPTION:
SC is a command line program used for communicating with the
NT Service Controller and services.
delete----------Deletes a service (from the registry).
Method 2 - use delserv
Download and use delserv command line utility. This is a legacy tool developed for Windows 2000. In current Window XP boxes this was superseded by sc described in method 1.
Method 3 - manually delete registry entries
Windows services are registered under the following registry key.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
Search for the sub-key with the service name under referred key and delete it.