Linux is usually much safer that windows. That's not true for ubuntu.
Here is a short text on how to hack ubuntu, please don't abuse it
When booting up select second item in menu -
recovery mode
Then type
passwd, type new password (twice),
reboot.
Now you can log in with that new password.
That's it, system is hacked in few very simple steps.
Only thing that might protect your PC is BIOS password and every kid knows how to remove that protection.
Other distributions, such as Fedora, are safe, because you need to know root (administrator) password to do this.