But just starting a process doesn't automatically associate it with the parent.

From the documentation that was available, that certainly used to be the case, indeed the MSDN used to shy away from using terms like "parent" and "child". Since Win2000 I wouldn't be so sure, although it does depend on what you mean by associate.

The history of Win32 APIs to iterate through processes is checkered, there used to be at least two rival systems, or even browsing HKEY_PERFORMANCE_DATA. However from W2k there was this (code extract from an XS module):

static int getppid(void) {

    HANDLE hToolSnapshot;
    PROCESSENTRY32 Pe32 = {0};
    BOOL bResult;
    DWORD PID;
    DWORD PPID = 0;
    
    PID = GetCurrentProcessId ();

    hToolSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
    if (hToolSnapshot == INVALID_HANDLE_VALUE) {
        return 0;
    }

    Pe32.dwSize = sizeof(PROCESSENTRY32); 

    bResult = Process32First(hToolSnapshot, &Pe32); 

    if (!bResult) 
        return 0;
    
    while ( Process32Next(hToolSnapshot, &Pe32) ) {
        if (Pe32.th32ProcessID == PID) {
            PPID = Pe32.th32ParentProcessID;
            break;
        }
    }

    /* Expected */
    if (GetLastError() == ERROR_NO_MORE_FILES) {
        SetLastError(ERROR_SUCCESS);
    }

    return PPID;
}
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