It helps to state the make & model of your system, not just so you can get an answer, but also so that others facing the same problem can be better informed.
I would not recommend Unetbootin. In my experience it does not work with Debian 7 or its derivatives.
dd if=/... of=/... bs=4M; sync is the best way to write the iso to usb pendrive.
Boot to usb by adjusting your boot device order in the BIOS menu (usually accessed by hitting 'F2' at power-on).
At GRUB menu:
Scroll down to 'Install'.
Press 'tab' key.
At bottom of screen, some text will appear.
At end of text:
Press 'space' key.
Enter (without quotes) 'vga=normal fb=false'.
Press 'enter' key.
The installer should now initialise.
This works for EeePC 701 series.