Hard to know, especially with a dated operating system. You did not mention the VP version being used.
If it's a recent release, go to OPTIONS | EDITING tab and in the Advanced section at the bottom choose NO (CPU only) and see if that helps.
Under OPTIONS | DISK tab click Clear Unused Cache Files, then restart the program and load your project.
Try a reinstall of your old version. Many legacy releases are available here
Be sure that the disc partition used for caching has ample free space. 
Confirm that the video driver and Direct-X are as current as possible. https://www.nchsoftware.com/videopad/kb/1772.html