A write error usually happens because a file is readonly and Speed Startup is trying to write to that file. Vista has a bad habit of changing files to readonly for not reason.
The error message usually tells what file is causing the write error. If the file is speedstartup.ini, go to the Speed Startup folder in C:\Program Files (do this by right clicking on Start and selecting Explore from the popup menu). Right click on the speedstartup.ini file, select Properties from the popup menu, and uncheck the Readonly checkbox. Also do this for the folder (the folder will always show as readonly but you still need to uncheck it once).
If the file is not speedstartup.ini do the above instructions for the file shown.