Probably the most useful tool in your arsenal is a Live Linux environment on a bootable USB drive, with persistence enabled.

Running a live version of Linux from a bootable USB drive allows you to boot virtually any PC (not Mac) regardless of the condition of the hard drive and / or Windows installation. You'll be surprised at what you can do when you're in a Live Ubuntu environment, that's not affecting the rest of your computer in any serious way.

Essentially, it is a standalone operating system, installed to a USB drive, that you can boot up in seconds, make major changes and repairs to a Windows installation on your hard drive and reboot the computer without any evidence you were ever in another operating system.

Say you have encountered a classic Windows XP error during boot:

"Windows cannot find the file C:\WIndows\System32\Config\SYSTEM"

Using your Live Ubuntu, you can boot into a temporary operating system, mount your internal drive, and repair/replace the corrupted SYSTEM file (which is actually a major component of the Windows system registry. Provided you have a clean SYSTEM file or a backup, you can boot Ubuntu from your USB drive, repair/replace the file and boot back into Windows, in about 5-10 minutes. Windows will never know the difference.

There are other features you can use here, such as CHNTPW which can help you reset the password on your Windows account.