unable to install any kind of software from Microsoft, licensed, etc in Windows 7

I am using an old PC that has quad core processor in it with 4gb of ram. All microsoft suite, adobe software, etc; have been installed from ages (1 year) now and everything used to work fine. But just day before yesterday I was not able to install any software whether it is from Microsoft or it is a licensed software? There is some kind of error with the application installer that is not able to write to the registry? How can I solve this problem?

Can you try to repair your Windows 7 System files with the help of System File Checker and see if that works? You will need to open an elevated command prompt by right clicking on command prompt and running it as admin. After that to repair the system files you need to type sfc /scannow and press Enter. When you are done then close the elevated comman prompt and then see to install any applications.

If the above doesnt work then type CMD in Search box and right click on cmd.exe and then run it as administrator. After when the command prompt opens, then type the below command and press enter one by one:
%windir%\system32\msiexec.exe /unregister then press enter
%windir%\syswow64\msiexec.exe /unregister then press enter
%windir%\system32\msiexec.exe /regserver then press enter
%windir%\syswow64\msiexec.exe /regserver then press enter

After that you can restart your pc and test to install any software.

This problem might be happening because you might have some conflict with some security software that is installed in your computer. Have you by any chance installed any trial version of Trend Micro, McAfee, Norton, etc? If yes then try to uninstall it and then try to install any software that you want. These softwares might be blocking the installer to install any software in your pc.