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    <p>It is really possible. The main problem which you have is that Windows should be seen as a terminal server and a users session as a remote session. Your service should be able to start a process which run in the remote session belongs to the user.</p> <p>By the way, if you write a service which run under Windows XP which is not added to a domain and the fast user switching is activated, you can have the same problems to start a process on running on the second (third and so on) logged users desktop.</p> <p>I hope you have a user token, which you receive for example with respect of impersonation or you have a <code>dwSessionId</code> of session. If you don't have it you can try use some WTS-function (Remote Desktop Services API <a href="http://msdn.microsoft.com/en-us/library/aa383464.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa383464.aspx</a>, for example <code>WTSEnumerateProcesses</code> or <code>WTSGetActiveConsoleSessionId</code>) or LSA-API to find out the corresponding users session (<code>LsaEnumerateLogonSessions</code> see <a href="http://msdn.microsoft.com/en-us/library/aa378275.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa378275.aspx</a> and <code>LsaGetLogonSessionData</code> see <a href="http://msdn.microsoft.com/en-us/library/aa378290.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa378290.aspx</a>) or <code>ProcessIdToSessionId</code> (see <a href="http://msdn.microsoft.com/en-us/library/aa382990.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa382990.aspx</a>).</p> <p>You can use <code>GetTokenInformation</code> function with the parameter <code>TokenSessionId</code> (see <a href="http://msdn.microsoft.com/en-us/library/aa446671.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa446671.aspx</a>) to receive the session id <code>dwSessionId</code> of the users session if you knows the users token <code>hClient</code>.</p> <pre><code>BOOL bSuccess; HANDLE hProcessToken = NULL, hNewProcessToken = NULL; DWORD dwSessionId, cbReturnLength; bSuccess = GetTokenInformation (hClient, TokenSessionId, &amp;dwSessionId, sizeof(DWORD), &amp;cbReturnLength); bSuccess = OpenProcessToken (GetCurrentProcess(), MAXIMUM_ALLOWED, &amp;hProcessToken); bSuccess = DuplicateTokenEx (hProcessToken, MAXIMUM_ALLOWED, NULL, SecurityImpersonation, TokenPrimary, &amp;hNewProcessToken); EnablePrivilege (SE_TCB_NAME); bSuccess = SetTokenInformation (hNewProcessToken, TokenSessionId, &amp;dwSessionId, sizeof(DWORD)); bSuccess = CreateProcessAsUser (hNewProcessToken, NULL, szCommandToExecute, ...); </code></pre> <p>This code only a schema. <code>EnablePrivilege</code> is a simple function used <code>AdjustTokenPrivileges</code> to enable <code>SE_TCB_NAME</code> privilege (see <a href="http://msdn.microsoft.com/en-us/library/aa446619.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/aa446619.aspx</a> as a template). It is important that the process from which you are start a process you have TCB privilege, but if your service run under the Local System you have enough permissions. By the way, following code fragment work with not only Local System account, but the account must have <code>SE_TCB_NAME</code> privilege to be able to switch current terminal server session.</p> <p>One more remark. In the code above we start new process with the same account as the current process have (for example Local System). You change change a code to use another account for example the users token <code>hClient</code>. It is only important to have a <code>primary token</code>. If you have an impersonation token you can convert it to the primary token exactly like in the code above.</p> <p>In the <code>STARTUPINFO</code> structure used in <code>CreateProcessAsUser</code> you should use <code>lpDesktop =</code> WinSta0\Default".</p> <p>Depend on your requirements it could be also needed to use <code>CreateEnvironmentBlock</code> to create an new environment block that you will be passing to the new process.</p> <p>I recommend you also to read <a href="https://stackoverflow.com/questions/3100934/how-to-ensure-process-window-launched-by-process-startprocessstartinfo-has-focu/3101061#3101061">How to ensure process window launched by Process.Start(ProcessStartInfo) has focus of all Forms?</a> where I describe how to force that the process will be started in foreground on the users desktop.</p>
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    1. COI've just been working through an issue I was having with a service that is doing this, and found your answer here... Very helpful. In my case, I was getting an annoying "access denied" error when I ran a file-open dialog from my spawned process (otherwise the dialog worked fine). Calling `CreateEnvironmentBlock` and passing the result to `CreateProcessAsUser` seems to have fixed it. Do I really need to set that `lpDesktop` value of my `STARTUPINFO` struct? I just left it as NULL. Also, it worked without setting that SE_TCB_NAME privilege.
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    2. CO@paddy: Sorry, but all the steps are really required if you have some common case. If the service run under some user account and not as System you really have to set `TokenSessionId` to enable the interactive user to see the windows of the started process. To be able to use `SetTokenInformation` with `TokenSessionId` the service account really need to have `SE_TCB_NAME` privilege and have to enable it. If the Screen Saver work at the time of process starting one should set `lpDesktop` to spectfy either the desktop of screen saver or default user desktop.
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