Hello Phil, and welcome to the Bleeping Computer Community. This probably isn't the most appropriate Forum, but the Moderators will move this post, if they deem that appropriate.
If I were you, I would download a copy Windirstat from here and run it. It will show you what is consuming your hard disk space. From what I understand, it is advised to keep at least 15% of your primary OS partition free for Windows and app temporary files, so you may be noticing some performance degradation.
If the culprit is something malevolent, then you could ask to get this thread posted over to the "Am I Infected" Forum to seek assistance there.
Hope this helps. Let us know how you make out, and once again, welcome to Bleeping Computer.