Defragmentation and Scan Disk cannot be completed

Hi Guys,
My name is Harold and I am having an issue with my computer. It is really slow now and according to my friends, I have to delete old and unnecessary files as well as defrag my computer. And that, I am willing to try.
My computer is running on Windows 98 and I have managed to tone down my files in the PC. My problem is, when I am defragging it will not complete the process. Even if I do scan disk, it is the same thing. I have tried to disable my anti-virus software, but to no luck.
Any other ideas you have in mind?
Thank you .
