just take the sd card out, turn on the unit, and turn off the autorun setting, then sort the card out
and just so its totally clear, sorry if you know this...
the file you download from the archive is a 'zip' file. This is just a way of 'packaging' the whole thing up to send across the internet. Inside the .zip file is a number of other files.
You must open up the zip file, then open up the gmenu2x-0.9 fiolder thats inside, then copy the autorun.gpu file and the gmenu2x folder that you find there, straight into the root of your SD card.
If you just have the gmenu2x.zip file on the root of you card, it will not work. If your autorun.gpu file is inside a gmenu2x folder, it will not work.
On your SD card you should see: autorun.gpu, and a folder called gmenu2x.
Note: Depending on your windows settings, you might not see the .zip extension on the zip file.