I suppose that you're using an nvidia graphics card. This is a bug I encountered as well...
Instead of loging in graphically, press [ctrl]+[alt]+[f1] in order to access the console login (as root).
There, you open your /etc/X11/xorg.conf file with your favorite editor, look for the line:
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Source code
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1
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Option "RenderAccel" "true"
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and set it to false. For me this helped, with the latest nvidia drivers (I now use version 8174) you can set back the option and it works fine, even with transparency.