Letter does not work on keyboard
You can try using compressed air can to clean the keyboard, maybe that will fix it. But if it's caused by wear and tear or you have spilled some liquid on the keyboard, you need to buy a replacement.
A keyboard for your laptop is around 17$ with free shipping:
http://www.ebay.com/itm/NEW-Keyboard-for-Toshiba-Satellite-A105-S2051-A135-S4447-A75-S209-M55-S325-Black-/300780897828?pt=PCA_Mice_Trackballs&hash=item4607f04624
driver needed for the intel 915gm chipset for toshiba model m55-s325
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