80,000,000,000 / (1024^3) = 74.5

Newer Dells have a recovery partition. That's where your space went. Check Disk Management.

There are some system files that cannot be moved by defrag. That is normal. I hope you are not using the built-in defragger. Defragging doesn't do much for performance anyway.

You need to look at what processes are running at startup. Sounds like something is eating up a lot of resources when you login. Spybot is rubbish, so I hope you are running better antispyware.