Its the Windows C+++++ crap. When you install Solidworks the installer tries to overide certain files but other and windows applications use these files sometimes and that interfers with the Solidworks Installer. No matter what you do from now on you will get same error message over and over again. When I installed my old copy I had to reformat my hard drive 3 times till I got a full installation going. I think they released a fix to this problem for some people it works for others it doesnt. The safest bet I think you will have to reformat reinstall windows then try to do a clean installation. Best practice when isntalling Solidworks disable all Antivirus, all networking and pc tools and other applications. Sometimes Antivirus interfers and prevents the installation Norton is famous at this
The Administration forum is the best for getting answers to installation problems. It's also a good idea to ask your VAR for help.