Allright there it is. My 80 gig is nealry filled up with many things but the problem is that all my stuff doesn't add up to 80 but to 60 gig. So there is 10 gig right there floating in the sky. I scanned my computer with avast antivirus and it showd me the names of these folders containing my space. Though I cannot find them in the search option of windows...
How can I access and delete them to finally have my rightful 10 gig?
What OS are you using and did you have Show hidden files and folder enabled and Hide protected operating system files disabled when you searched for the files?
The three main folders you are going to want to check are "Documents and Settings", "Program Files", and "Windows".
If these three folders equal about 70 GB or so, you probably aren't missing any space. But, if you still are missing about 10 GBs, it is possible that you have some DVD backups stored somewhere on your HD (assuming you burn DVDs). I've seen this on a few computers that I've worked on, and CCleaner doesn't get rid of it unless the app is specified, or the movie is stored in the temp folder. Apps like DVD Fab are notorious for this.
And, of course, you've been running CCleaner to get rid of temporary crap on your computer, right?
If you are using Win XP with System Restore enabled, the 4th folder to check is System Volume Information. The default allocation of 12% X 74.5 GB = 8.9 GB.