In the small application that I\'m currently developing for a customer I need to ask the user for his windows login username, password and domain and then use those with
Try looking at the SecurePasswordTextBox custom control. Are you trying to do something similar to a "Run As" type command where you are trying to run the process as a different user than the one currently logged on? If not, you should be able to just call Process.Start and let it pick up the current users credentials.
Also, take a look at the following resources as well:
The best option would probably be to use some interop p/inovke code to call CredUIPromptForCredentials to display the standard Windows dialog box and then use that information to either call Process.Start, or, more likely, call the CreateProcessAsUser function.