Follow these steps to convert MBOX to PST:
- Launch Kernel for MBOX to PST software.
- Select MBOX file(s) or add the folder containing MBOX files.
- Check the Preview of all mailbox folders.
- Choose the Output file format from the list of formats.
- For selective migration, use data filters...
Watermarks are there to help the maker of the video to retain his rights over the video he created.
When you choose to use that video I think you need not erase the watermark because one needs to give credit where its due.
If you want lightweight antivirus for your system you can use the built-in Windows defender.
But it may not protect your system very well.
You could also try Avast or webroot.
Both these give average security and are lightweight.