Chances are that you do not have the CUDA miner executable or that GUIMiner is pointing to the wrong location. Open up explorer and check the path that is set in Ext. Path (In GUI Miner) to verify that the executable is there.
An easier means to an end is to use something like CG Miner or OpenCL Miner. Both should be available out of the box in GUIMiner.
If you get going and you like how it works though, I'd suggest getting more powerful mining hardware.
The problem was exactly that! Wrong path location
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