Sampling can be used on any code that runs on the system: application, driver, service, etc. As long as you have debug information for the module (i.e., exe, dll, etc.), the analyzer can display hotspots within your code.
To use call graph on a Windows service, there are special instructions. Please see http://support.intel.com/support/performancetools/vtune/sb/cs-006207-prd232.htm.
The service cannow be call graphed using the following steps: