Windows 7: computer's life time

I am going to take a different approach to your question.
Assuming that it has been used before, the only component that would possibly have a decreased life expectancy is the Hard Drive, since it the only moving part. All other components do not wear out, as does the Hard Drive; although they can, of course break. Run the chkdsk function on your computer and then defrag; if all is ok just enjoy. Any software can be downloaded again, if problems develop.
I do wish you the best of luck with your computer.

You can use SMART tools to see how many times your hard drive has been powered on and the total number of hours it has been in operation.

I knew about that and totally did not think of it. +1 there.