This occurs when a Windows 7 system has been sysprepped three times — KB929828. To overcome this, you will need to do the following:
Run the slmgr.vbs -rearm command.
Try sysprep again.
Additional ressources :
http://technet.microsoft.com/en-us/library/dd744512%28WS.10%29.aspx
http://mickitblog.blogspot.ca/2011/08/fatal-error-occurred-while-trying-to.html