I need to use a folder lock software to protect my private folders. But there is a main problem with most of them.
Imagine I installed one of them on my sysem that runs Windows 7. Everything is ok up to now. But if I lose my computer and someone else reinstalls its OS or uses another operating system like Ubuntu on it, files are accessible.
What do I have to do ? Is there any suitable software?
You are asking the wrong question. A folder is simply a logical construct on a filesystem. A more appropriate question to ask is what filesystem/storage mechanism (which allows folders).
There are probably lots of answers to this question - they will all involve encryption. A good, free, cross-platform answer is "Truecrypt". Truecrypt will allow you to encrypt a physical disk or a file on your existing disk into a virtul disk - which gives you what you want.
Why not use a password protected ZIP file? This is a much simpler solution.
Encryption comes with Windows and one of the "features" is that losing OS means losing data, unless you backup your recovery key.
So all you need to do is to right click on folder and select Properties -> Advanced -> and check "Encrypt contents to secure data", include subfolders and files.
One of the advantages is that you don't need to remember another password, all you have to know is your Windows account password only.
@davidgo is correct in that encryption is the solution and I agree that Truecrypt's full disk encryption is the ultimate solution.
If you don't want to go to that extent, Axcrypt allows encryption on a file and folder level which too would deny access to your private folder(s) and files from anyone booting to, say a Live-CD as you described.
Axcrypt, unlike Truecrypt, is currently "Windows Only" which is a problem if you need access to your folders/files from other platforms (e.g., Mac OS X or Linux).
