I installed Ubuntu 12.04 on a laptop from a USB stick, but the system asks me a password and I don't know what it is. It may be old to have an Ubuntu installed and errors, but I can't remove, because Windows XP is damaged and unbootable. Someone can help me?
What you should do depends on what you mean by "Uninstall Ubuntu". There really is no such thing as an uninstall. You could format the hard drive, which would be the closest thing. This would remove everything from the computer, including ubuntu. If you want Windows back, you can just insert a Windows installation disc and install it. Delete any existing partitions that you see, and it will automatically overwrite ubuntu.
If Windows XP was working fine before you installed ubuntu, it might still be bootable. Hold down shift while powering on you computer and keep holding it. You should see the GRUB menu. Chances are, Windows is in that list.