Windows 7 SP3 Installation

You're misreading something.  There is no Service Pack 3 for Windows 7.  In fact, there's no Service Pack 2.

See "Step 6" here, especially the 5th paragraph:  https://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install-wininstalls/clean-reinstall-windows-7/0ae269fd-167e-423b-94ef-26133f89e10c

If following the instructions in Step 6 don't result in getting Windows 7 SP1, see:  https://support.microsoft.com/en-us/help/15090/windows-7-install-service-pack-1-sp1

You want either

windows6.1-KB976932-X64.exe if you have a 64-bit system

windows6.1-KB976932-X86.exe if you have a 32-bit system

Keeping in mind that we can't see what you are seeing it might behoove you to use the instructions in this link to include a screenshot in your posts:

How to include a screenshot in your post

I checked my Installed updates and filtered for service and saw this which show a Service Pack 3 for Office, but the only Service Pack for Windows 7 is KB976932 (Service Pack 1):

First of all, thank you very much for your kind answer. Just after posting the question I just realized that it was Service Pack 3 for Microsoft Office, and not Windows, but I didn't know how to delete the question. Finally, I had to install Service Pack 1 manually since it didn't do it automatically, but everything is ok now. Again thank you very much for your help and sorry for the inconvenience.

Carlos.