I think you have got some worng information. First of all .NET Runtime is NOT installed on Windows XP. It is bundled in Windows XP Server Pack 2. So on a fresh Windows XP system you will have to manually install .NET Runtime.
The only OS on which .NET runtime is installed by default is Windows Server 2003.

Secondly the .NET runtime does not run as a service by default. You can start any Managed (.NET Application) and the platform will load automatically provided the correct runtime is installed.