does windows really take up 20.5gb?
Yes. You can trim that some, but standard Windows installs are around 20 GB.
how come my seagate doesn't say 500gb of 500gb? where did the 35gig go?...
Because they use a different definition of gigabyte than your computer does.
Your computer says a gigabyte is 2^30 bytes. Hard drive manufacturers say it's 10^9. The result is your computer reports less space than you expect. It's been this way for literally decades. Very large drives will be "missing" literally hundreds of gigabytes of space.