My 91-year-old, father-in-law's computer's hard drive was too full (65 of 70GB). I was looking through his folder's for high content, and under Windows found 24GB of $NtUninstall !! This can't be normal. We're going to get an external hard drive to expand his capacity, but wonder why this happened at all and how to prevent it continuing. (At 91, he is not up to learning a new computer, or anything complicated, and as his primary care giver I don't have the time )
These folders contain the uninstall information for each Microsoft update you install. That means that you will be unable to uninstall the updates if you delete these folders.
If your system is stable, however, and if you no longer plan to uninstall an update, then go ahead and remove the corresponding $NtUninstall folders.
Or you may move those folders to a different hard drive, or burn them to CD before deleting them from the hard drive.
... and have a look at the amount of space reserved for the Recycle Bin. MS tend to give it the equivalent of a skip bin inside your front room, as opposed to a waste-paper basket.