A Google search seems to indicate that these are some sort of project files -- likely from a Sonic product.
Do you have a program like DVDit Pro or other Sonic product?
Though other searches indicate it is a session file, the result of your having more than one instance of a program open at once.
And others indicate it is related to an old program called Cool Edit Pro or Adobe Audition.
In any event, it is specific to a program, and not a graphics or audio file that you will be able to convert.
>converted old vhs tapes to the pc
How did you do this... what equipment and software did you use?
When I use a Pinnacle Dv500 card to digitize tapes, the files are DV AVI type 2 with 16bit 48khz sound
SES files are usually "session" files, and are generally associated with Audio programs, like Adobe Audition, as Steve says.
Now, it does seem that either Magix, or CyberLink NLE's also do SES files. Do either MovieEdit Pro, or ProwerDirector figure into the workflow?
As John T. asks, can you please tell us the exact steps of your workflow to get to the SES files?