The key here is to use OpenProcess, EnumProcessModules and GetModuleBaseName. Assume that the target Process ID is in the value dwPid, which is a DWORD:

HANDLE hProc;
char szProcessName [80];
HMODULE ahMod [10];
DWORD dwNeeded;

hProc = OpenProcess (PROCESS_QUERY_INFORMATION|PROCESS_VM_READ,
                     FALSE,
                     dwPid);
if (hProc)
{
   if (EnumProcessModules (hProc, 
                           ahMod,
                           sizeof(ahMod),
                           &dwNeeded))
   {
      if (GetModuleBaseName (hProc, 
                             ahMod[0],
                             szProcessName,
                             sizeof(szProcessName)))
      {
         <success>
      }
      else
      {
         <failure>
      } 
   }
   CloseHandle (hProc);
}

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