The solution would be to use
->Parity Archive Tool, but somebody's cooperation is required. In windows you can use
QuickPar.
I assume You are downloading windows version.
First You MUST free that file from "use by another program". Try to wait several minutes:
- after opening folder containing downloaded file
- or after starting the system if the file is on the desktop.
If the file still can not be renamed or moved get the
Process Explorer (scroll to the bottom and choose appropriate version), find the process wich holds the file open and end it (or kill it).
Later today I'll post here a link to small (several kB) parity file. After downloading it, with help of QuickPar you will be able to check integrity of downloaded PS installer. As a result You will get the number of broken blocks/bytes, post those informations here. I'll then post a link to smallest possible file (or set of files) required for repair (on the condition, that it will be resonable small amount of data). Then You should be able to repair Your installer.
I am not sure, if I still have windows installer on my disk at home, so if somebody else can help with that procedure sooner, be my guest.