The withdraw button is misleading. It's for banning guidie, a right you don't have. 2 ways to get out of guild:
Tell your Guild Master (GM) to bann you while you're off line so you don't have to wait for 3 days to join another one.
Leave by urself: look here
http://www.silkroadforums.com/viewtopic.php?t=230 for the Guild Master NPCs in any town. He can help you leave but you have to wait 3 days to join another guild.