You can download TreeSize Free - Quickly Scan Directory Sizes and Find Space Hogs to see what is taking up space. Most likely system and hidden files are making up the remainder of the space. I would recommned taking a screenshot of the top directories taking up space. Don't just delete random items or you could break Windows.
Also you can try running the disk cleanup utility which will delete items that are safe to remove.
To expand on Neemobeer's suggestion: - The first run of the "Disk clean-up" scan does not find files that have accumulated over months and years of Windows updates. These files are no longer needed but remain on the computer. To find these requires the following procedure: -
1. Tye "Disk clean-up" in the run box and hit the "Enter" key to open the program. Then click on OK to run it the first time.
2. After the search is complete do NOT click on "OK". Click on "Clean up system files" instead and then click OK. This will cause it to run a second time and it will take longer and find a lot more files to delete. These are the files I mentioned above.
3. When second search is finished click on "OK" then click on "Delete Files" to start the delete process. The message makes it clear that the files being deleted are "Unnecessary" so it safe to delete them. This operation can be cancelled if it is taking too long and you wish to do it another time.
4. The first run may find only 50MB to delete but the second run may find 20GB to delete if this has never been done before.