I suggest you run Glary Utilities, particularly the Disk Analysis tool (under the Files & Folders tab).
You may just have a load of temporary files, which can be easily removed with GU's "one-click maintenance" option.
Filling up the hard disk beyond about 75% of capacity slows your machine down and strongly increases the chances of a disk failure. You should really buy some sort of external HDD for backup purposes — something like this would do the job, although I'd recommend 320GB/500GB — you could put all your media on it and keep your laptop's hard drive for documents and program files.
A 100GB hard drive actually contains no more than 93GB of 8-bit (power of 2) addressable storage, and your operating system alone probably takes up 10GB or 15GB of that, leaving less than 80GB. So your iTunes collection, at 19GB(?) is using up at least a quarter of your HDD's capacity.
Try that GU utility and you can get an exact breakdown.