Strange / Weird Problem

[Iced_Bullet]

It says I have filled around 50% of my 1tb HDD shown here:

but when I highlight everything inside it, it only shows that it's 305gb:

Am I being completly brain dead and missing something obvious? Or is something wrong? btw, it was installed 4 weeks ago when I built my new PC.

 

[johnb35]

Make sure that you have "show hidden files and folders" checked

 

[Iced_Bullet]

I did, that's why you can see 'Program Data' and the symbol is slightly faded.