I am using an old Acer Aspire 4315 and operating on Windows Vista. The C drive is at full capacity. I have moved all data to the D drive and erased non essential programmes. This has had no impact. I went through item by item listed under C Drive and added up the size of each item and this totalled only 17G of the 32G capacity. The machine meets my needs. Any suggestions to fix?
The usual suspects, such as the page file, system restore points...
I suggest that you download a copy of WinDirStat and run it from a privileged account. Turn on the option to show hidden / unknown space, and it will tell you where everything is. If you use an image-serving service like Photobucket, you can post the image here.
Finally, unasked-for advice. The machine does not meet your needs, it has run out of space. Why not spring for a 100 GB or larger drive, clone your system drive to it, and not worry about system drive space for the next five years?