I checked and the entire ArcGIS_Security folder has read/write extended to IUSRS account.
Have you modified the ViewerContainer.mxml ?
http://forums.arcgis.com/threads/25899-Login-Screen-for-the-Flexviewer?p=139178&viewfull=1#post13917...
Just posted some code for a Flexviewer login screen. I hope somebody can get some use from this.
http://www.arcgis.com/home/item.html?id=baebcaf317994d63902bc9735c0657e0
Hello We cannot access the link