To be perfectly honest, if you only have 500MB of free space on your C: drive I think you'll end up with serious issues sooner or later. It's also generally a not a great idea to install software on external drives for many reasons, and it would be preferable to use another internal drive, if possible.
Hard drives are very affordable these days, and unless you have a laptop, it should be easy and inexpensive to clone your existing drive onto a much bigger SSD and get lots of performance boost to go along with your new capacity. You could also partition it into C: and D: drives and use the D: drive for videos and documents while using the C: drive just for the OS and programs/apps.
If you can't upgrade your C: drive, you can create a junction linking the C:\Program Files\CyberLink\PowerDirector18 folder to a new folder on your external drive, so when you install PD, the installer thinks it's going onto the C: drive but Windows is actually copying all the files to the external drive.
Use
mklink /J "C:\Program Files\CyberLink\PowerDirector18" "x:\PD18" (where x: is your external drive's letter). See this
link, or Google if you need more info.
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