In order to transfer music files to your new desktop computer, first I would recommend you copy the files from your old computer to a external device like Pen drive, Flash drive or external hard drive. You can then connect the device to your new computer and past the files to your new computer's hard drive.
Once the files are copied to the new computer's hard drive, you can use the scan disk for media which will import the file to the RealPlayer PC library.
Please note that doing this will only retain the original meta data associated with the files, which means if you have updated any information like Track name, Artist, etc in the RealPlayer Library then it will not be carried over. In the same way, the playlists needs to be recreated.
To know more on this, please click this link: https://customer.real.com/hc/en-us/articles/204039313
I hope this information will help you.