Steps to do this should be documented in your Users Guide. You can get one for your model here:
http://www.csd.toshiba.com/cgi-bin/t...su_sc_home.jsp
Try using the System Restore Disc and boot from it (CD/DVD drive). It should give you options on what to do next. Also see steps here:
http://www.bleepingcomputer.com/tuto...torial148.html