How can I get back to my old windows7 from Windows.old?

May 22, 2018
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I have re installed my windows 7 today.But I had some file encrypted.Now,I can't access those files.I've tried to decrypt but it says,"Access Denied".Searching on internet,I come to know that I will need a key.but as I reinsalled win7,the key is no more in my device.Now,there is a file named WINDOWS.OLD in my C drive.How can I go back to my previous Windows 7 so that I can decrypt files?
If I can't go back,how can I decrypt my files?My priceless data are on that file.Those worth my life.Please help me!!!