The free space in my drive keeps shrinking though I haven't installed any new legal software in it.
Is this a Vista OS problem, or is this some sort of malware or virus engineered to scrape data fragments off my system drive?
Most likely it's due to Hard Drive failure as most Vista computers are nearing 10 years old this year (from 2007); and Hard Drives are only designed to work for 3 years in Desktop PCs and only 2 years in Laptops!
If you've owned that computer since it was new and never had the Hard Drive replaced; it's 90%+ certain that it's failing or now completely failed!
After you try to upload your Disk Usage log to neemo; consider testing your Hard Drive and RAM sticks; replacing them if failure occurs. I have written a thorough step-by-step Troubleshooting Guide to assist you with the HW Testing here: Windows 10 - Unclickable Task Bar
The Guide is quite long (6-8 pages), and so is a good idea to print out and have hardcopy in front of your computer while attempting to Troubleshoot it.