First, download and run ccleaner from www.ccleaner.com. Look for the
Install it and let it analyse. This will tell you how much space is used
by unneeded files, very few of which will be in the Windows folder
structures. Note that if you let it remove cookies in the "cleanup" phase,
it will likely remove any stored information you have for thinks like bank
cards and site passwords.
The first time this runs, it's not unusual for it to find over a gigabyte of
files that the Windows tools leave behind.
However, if this amount of space makes a difference to you, you should
consider upgrading the drive to something significantly larger. This is
fortunately easy to do and not expensive - where I am, half-terabyte SATA
desktop drives are around CDN$70, one-terabyte drives are about CDN$160.
You just need a way to connect the drive to your system (such as a USB2
drive case, starting around $25) and some cloning software, such as the
Acronis TrueImage free trial.
Run this in Manual mode, so that you get the option to do a proportional
clone, meaning that the partition sizes will be expanded to the size
available on the new disk.
You'll be done in a couple of hours, most of which will be spent doing
something else. When the cloning is done, simply remove your old disk and
install the new one exactly as the old one was in terms of cabling and any
jumper settings. The only difference you should notice is that you now
have more than enough space.