Now for the best solution, use KonBoot. You still have to burn the ISO to a cd or write it to a USB drive and boot into the image like you would ophcrack but its how it works that is pretty amazing. What it does is it takes the Windows Kernel rewrites it and starts up Windows like it normally would. Only this time when you are prompted for a username and password just hit "enter" and thats it you are in. Here is the catch, you can not change the user password but can use it as if you were the user. It pretty much spoofs the users credentials and lets you go from there. This has been tested on all Windows version (XP - 7 & 2000 - 2008)

Now if you wanted to be malicious (ninja style) you could have this on a 1gb thumb drive and be able to access any PC simply by booting into the USB and there would be no trace of you ever being there.