There are no Model C640's sold in the U.S. and as mentioned above several times, this is the site for U.S. machines. If your model IS a U.S. machine, click on Downloads above and follow directions to find your drivers and other support. If it is not a U.S. machine, contact Toshiba in your part of the world.
C.B. Toshiba Sat. L675D-S7016 Win 8 PRO 64 Bit---Toshiba Sat. L775D-S7132 Win 7 PRO SP3 64 Bit which also has Virtual Machine with Win XP SP3---Toshiba Sat. L305-S5921 Win Vista SP2 32 Bit