OR... if you don't mind doing a bit more setup... you can create a new bitmap with CreateDIBSection (too lazy to get a link... just search MSDN for it) that way you have a pointer to the raw pixel data and can change it at will.
Put the loaded bitmap into a DC, put the DIBSection bitmap into a DC, blit/copy the loaded bitmap to the DIBSection (look up BitBlt)... then you can modify it through that pointer.
Don't forget to clean up. Any bitmaps that you created/loaded need to be destroyed with DeleteObject, and any DCs you created need to be destroyed with DeleteDC.