Download the trial of FMP 11 and test?
Post the question in the Parallels Forums? Create a support request on Parallels site?
Call FileMaker Tech Support? From what has bee posted here by TS personnel, FileMaker Inc. doesn't charge for installation support.
Just got off the phone with FileMaker Support and the tech got me going; the problem is classified under "We don't know why this happens, but it's Windows." Anyway, here's the skinny on getting FMPA 12.x to load in Parallels (with Win 8):
Instead of double-clicking the Setup.exe file in the top-level FileMaker Pro 12v3 Advanced folder, open the sub-folder, Files, then double-click its Setup.exe file. *Poof!* It works.