I'm afraid the amount of shear crud that windows patches (aka updates) that MS forces on you is undeniably huge. Every patch has an uninstall routine and a log left behind. In no time these can add up to a Gb of useless space. A service pack tops out at 1 or more Gb. You can only see these by being an administrator user, and setting options to view system files. That is why so much space just seems 'lost'. If you know what you are doing patch logs etc can be deleted without bad effects
On a 35Gb machine, you are never going to have any space. Look into buying a new drive at once. It would have to be an IDE drive (I suspect, dont trust me on that, some 9400 can go up to 320Gb sata drives), you should be able to get one at least 80Gb for the price of a good dinner, and if new, it should come with a utility to clone your existing drive onto it, otherwise you will need some slightly specialised software (but still free). Sadly, that act of cloning is much less straightforward on a laptop than a desktop. It can be easier to take your HDD out and mount it in a knowledgable friends, along with a new big one, but special adapters are needed (very cheap). Cloning is then easy.
Unfortunately the whole alternative topic of clearing useless crud is a very large one. Look to start with anti-virus logs and updates, firewall logs, system restore points, have a fixed pagefile size and so on.