As Windows boots, you are allowed to press a number of hot keys -- like the F5 key to specify safe mode or the...
F8 key to specify boot options -- in order to have some control of how Windows loads. These keys produce three byte, key-code sequences to notify Windows to perform a specific startup action, all of which start with the Esc key.
When the Esc key is pressed, Windows detects this byte and waits for the additional bytes to finish the sequence, and it will not continue loading until the sequence is completed. To get past this situation, press another key to finish the sequence and let Windows continue to boot.