storage - windows xp, "lost" memory in c:windows folder

I'm running windows xp on this laptop, and experiencing something "weird".

My (minor, system) hard drive was once again getting full again. (it's a 30 GB drive). So I started to look into where the data is stored. I see 7 GB in the documents & settings folder, and 12 GB in the windows folder (c:\windows).

Now I just went to check the windows folder (not that I would change anything there), and notice something: when selecting all files and folders, and clicking on "properties" it only reports 8 GB

So I lost 4 gb somewhere? What can cause this? And yes I've setted windows to show hidden folders & system folders.

On an unrelated note: have you guys also experienced a strange influx of programs which write huge amounts of data to the %appdata% directory instead of where they are installed? Strange.