Hey there David,
I am not sure, where were they stored? -80C or 4C or -20 or at room temperature?
Anyways, I think you should take a look at the DNA you are extracting and see what you see on a gel. However, I bet you don't have that much so not sure if you want to run these on a gel to waste the DNA.
I would probably isolate something using what you are doing, and then try PCR on a small amount to see what you can get from a gene you are interested in.
There probably are better kits for extraction of DNA from slides....
Tell us what you find!