ITaskScheduler access denied error

I've developed a COM+ server component (dll) that uses the ITaskScheduler and ITask interfaces to create and edit tasks for a specific .exe also created by the company I work for. The component is called from a classic ASP page (VBScript) and is part of an office package we are developing. The whole system uses a web interface. When run under IIS on Windows Server 2003/2008 I get 0x80070005 access denied errors when attempting to call, for instance, ITaskScheduler->Enum. This makes perfect sense, the IUsr_... account shouldn't have access to the task scheduler. I added fields for the user to enter their credentials on the webpage, and then made calls to LogonUser and then ImpersonateLoggedOnUser in the COM object. However I still get access denied errors. Subsequent calls to IServerSecurity->QueryBlanket show that the COM object is still being run under the IUsr_... account. My logon logic is as follows:

bool SystemUser::LogonUser(const wchar_t* userName, const wchar_t* domain, const wchar_t* password)
{
    if(::LogonUser(userName, domain, password, LOGON32_LOGON_NETWORK, LOGON32_PROVIDER_DEFAULT, &_token))
    {
        return true;
    }
    System::LogSystemError(__W_FILE__, __W_FUNCTION__, __LINE__, L"Unable to logon user: %s domain: %s", userName, domain);
    return false;
}
     
bool SystemUser::Impersonate()
{
    if(::ImpersonateLoggedOnUser(_token))
    {
        return true;
    }
    System::LogSystemError(__W_FILE__, __W_FUNCTION__, __LINE__, L"Unable to impersonate user");
    return false;
}
     
SuccessCode::Enum SystemUser::Logon(const wchar_t* userName, const wchar_t* domain, const wchar_t* password)
{
    if(!_token)
    {
        if(!LogonUser(userName, domain, password) || !Impersonate())
        {
            return SuccessCode::ImpersonateError;
        }
        else
        {
            Global::systemLog.Write(LogLevel::Information, L"Successfully logged on as user: '%s' domain: '%s'", userName, domain);
        }
    }
    return SuccessCode::Success;
}

Using LOGON32_LOGON_INTERACTIVE as the logon type makes no difference. Neither does setting up specific roles in the COM+ MMC. Any help or suggestions hugely appreciated.

Tags: C++, ASP, COM, Scheduler, Impersonation, Task