I have the same problem. My machine is windows 7, 64 bit. I have the free version of SuperAntiSpyware 5.7.1018. I keep getting definitions update error.
Authenticating Connection is okay - I get a green tick and the word "complete"
Checking for definition update is okay - i get a green tick and the word "complete"
Downloading Updates (mirror 1) is okay - i get a green tick and the word "complete"
Please note Downloading Updates (mirror 1) happens 3 times for me.
At the end, I am told:
Definitions Update ERROR - failed. This section is all in red.
This has been going on for me for the past 1.5 hours.