which one is the best policy for using ubuntu? Installing inside windows using WUBI or install ubuntu directly booting from live cd?

If you want to play it safe Wubi is a good choice, but compared with a regular installation, a Wubi installation faces some limitations. Hibernation is not supported and the filesystem is more vulnerable to hard reboots[1]. Also, if the Windows drive is unmounted uncleanly (most commonly because of a Windows crash), Ubuntu will not be able to mount the Windows drive and boot until Windows has successfully booted and shut down. If the Windows system cannot be booted after the crash, the user also cannot boot Ubuntu.
Performance related to hard-disk access is also slightly slower, more so if the disk image file is fragmented, on a Wubi install compared to a normal one.

A LiveCD is another solution when you want to try Ubuntu without any changes to your machine!

Personally I would go for a regular installation and install it next to Windows so you can easily switch back when the need arises or if Ubuntu does not suit your needs. This guide might be helpful: https://help.ubuntu.com/community/GraphicalInstall