Jim,
My fresh VM of WIN7 64bit already has an association for .sch files. I points to schdpl32.exe, MS schedual+
I think you need to scan the registry for these extensions:
.sch .slb .mvl
And delete the entries. You'll find them in HKEY_CLASSES_ROOT and HKEY_CURRENT USER You might want to delete all the keys tha have psched.exe in them too.
Then reinstall Pspice.
My demo does not over write associations so it never set up the .sch association. There may be an additional key that makes every thing work that did not get setup.
Cheers