In Photoshop Elements 6, you should not be saving your slideshow as a WMV. You should use the Export to Premiere Elements function.
This sends the slideshow to Premiere Elements in a slideshow interchange format. Once it is in Premiere Elements, you can right-click on it and break it back into individual photos.
As for backing it up, it depends how large it is and if you want to save it as a video or as individual photos.
For Projects/Slideshows that are done in PE, you should be able to use the File>Archiver to do a Backup of the Project file (PREL) and the media Assets. There are two main choices and some settings. One choice is whether one wants only Assets that have been actually used in the Project, and the other is for ALL Asstets that have been Imported.
I would assume that with the SlideShow Interchange (in lieu of the WMV Export/Import), the SlideShow would also be saved from the Archiver.
I use a series of 1TB external to do the same from the Project Manager in PrPro. Many use external drive bays with hot-swappable SATA II HDD's. They'll fill these up, remove and store them, and just stick in an empty drive to that bay. Some remove the "carriage" from the HDD, but many just buy one of these for each HDD they own. Don't know of any mfgr. of hot-swappable ATA-300 HDD's, but am sure there are some. Fry's just had Seagate ATA-300 1TB HDD's for US$89. That would mean that the media for a 4x bay would cost less than US$400, including the hot-swappable carriages, and give one 4TB of storage at any time, plus infinite storage by swapping out the full HDD's. BD-Data discs would also be an option.
I do not know if you have covered this in one of your books, but, in case you have not, here is a caution when using the Premiere Elements Project Archiver/Archiver option that Hunt brought up in his response to this topic.
A user at another forum claimed that the Premiere Elements Project Archiver/Archiver option was creating corrupt files. As you will see in that thread, I think that I have found the cause and fix. I am not sure if I have all the whys.
Often you see the recommendation to convert problem video files into DV AVI in Windows Movie Maker prior to bringing them into Premiere Elements.
If you incorporate a video, that was converted to DV AVI in Windows Movie Maker, into a Premiere Elements project, and then decide to Project Archive the project, using the Archiver option, no go. This is not the case if you use the Copy option. Windows Movie Maker generates a Type 1 DV AVI. In contrast to Premiere Elements which apparently uses Type 2. To get the Project Archiver/Archiver to function under the circumstances described is was necessary to convert the Windows Movie Maker DV AVI Type 1 to Type 2 with DVDate. Again, this is not necessary if you select the Project Archiver Copy option instead of the Archiver (aka Trimmed) option.
Give it a look, see if you agree, and, if so, just keep that in mind when using the Project Archiver.
Thanks for the heads-up, Tony!