The quickest way to access an account is either to remove the password in safe mode, or if that isn't possible, booting from a CD/floppy and replacing the password in windows password file. Since I don't remember offhand, you'll have to google for where the password file is and what tools (if any) are needed to clear the password.
The drawback to this method is that any encrypted files on the computer will become unreadable, since the password is used in the encryption process. If you have encrypted files on the computer, you can't just replace the password, you have to crack it. Grab a copy of the password file, and then run some sort of crack program on it (I've never needed to use one, so google is probably your best bet to find a program to do this). Depending on the strength of the password used this could concievably take years to crack, so its can get painful if you used a difficult password. I'd use this method only if I really, really needed the encrypted files.
Note that both these methods require physical access to the computer. These types of attacks aren't really all that sensitive since once you give someone unsupervised access to the computer you're already screwed. If nothing else, they can always reinstall windows or poke around through your unencrypted files with a live CD. Based on your level of paranoia, there are a few things you can do to prevent people from accessing your files. First, of course, is to use windows encryption to encrypt everything you really care about.
Next, you should disable the storing of LM hashes in your windows password file. LM hashes are kept for compatibility with old versions of windows, but they're patheticly easy to break and once you break the LM hash the more secure "NT hash" used for current windows versions doesn't matter. Google "LM hash" for more info on what it is and how to disable it.
Third, you need to prevent the computer from being booted from a CD/floppy in the BIOS, and set a BIOS password so these settings can't be changed. Windows is pretty good about locking down its password file so that no one can access it from within the windows environment, but if you boot from a live CD or a floppy, thats no longer the case. If you force the computer to boot from its hard drive, this problem goes away.
Lastly, you need to prevent people from clearing the BIOS and resetting the BIOS password. This is why computer cases have loops for padlocks on them. Its not terribly difficult to reset the BIOS and clear/reset the password to its default, but it requires physical access to the inside of the case. Of course, people can always cut the lock off the case, but that makes is painfully obvious that your computer has been compromized.
Assuming you're paranoid enough to believe all these steps are necissary, someone who wanted your files would have to steal your box, cut off the padlock, reset the BIOS, find the default BIOS password on the internet, set the BIOS to boot from a CD/floppy, grab a copy of your password file, run a crack program against the stronger NT hashes, and only then would they be able to access your encrypted files. Most people wouldn't bother.
Personally, I use none of those methods, I just make sure that no one I don't trust has access to my computer, and leave it at that.