I think that you will not get away with a fully automatic solution (though I have not looked for them specifically).

The best way is probably to first check if your new program accepts your existing metadata.

Unfortunately, I also have no access to Windows Photo Viewer (WPV), and searching the internet proved inconclusive on whether WPV saves the metadata to the image file or in its own database. This is the first thing you have to find out, as the latter would make it really hard without some serious hacking (best case: it is a SQL database that one can read out - "just" read it and transform it into file-saved metadata, right? ;-) ) EDIT: WPV writes to EXIF's XPComment and XPLabel fields. Strikethrough text is unneccessary for WPV in particular, but might help others with different software but similar problems.

There are several options available to see image metadata:

• Most free image viewers, e.g. IrfanView or XnView offer metadata viewers (I in IrfanView) - they may limit what metadata you can see, however.
• CLI programs, e.g. exiftool or exiv2. E.g. exiftool: exiftool.exe -All:All <PATH\TO\IMAGE.EXT> - They will show you pretty much all metadata that your image file includes. There might also be GUI-frontends for them that will work, e.g. exiftoolgui.
• Free image organizers like DigiKam might offer a way to not only show metadata, but also save them in different standards.

If the metadata you are looking for can be found in the images, you can try to get them into a standardizes field, like IPTC's Keywords or XMP's Label - You will have to find out which field can take the information that you want to place there, and you also need to check if your new tool can read them in a way that is helpful to you.

In exiftool pseudocode, this copying could look like this (for more examples, look at the "copying examples" on exiftool's page):

exiftool.exe -<DEST_FIELD><<SRC_FIELD> "<FullSourceFileName>"

This answer to "Trying to Find a Windows “XP Comment” Editor" on StackExchange indicates that -XPComment and -XPKeywords might be the interesting tags for you.

There might be ways to do this with DigiKam or other tools, as well, but since exiftool is my go-to-tool for this kind of stuff, I really don't know.

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If those metadata cannot be found in the image files (and there is no way to save/export them inside WPV), then you will need to:

• Find the database path
• Find out what kind of database it is (e.g. SQL)
• Find a tool to read it out (maybe HeidiSQL?)
• Find the fields you need (most likely the image's path and your metadata)
• Find a way to save those entries either as sidecar-files or add them directly into the image files (I guess you will need more than one tool to do that)