What is the average "life expectancy" of today's PC?

This is a decent machine even by today's standards. Processors haven't gotten much faster of late, so you don't need to upgrade for that reason. There aren't many failure mechanisms; you are most likely to lose a disk drive because they are mechanical, but they have improved markedly over the years. I lost one machine due to a motherboard failure of some sort, but that isn't common. Bottom line: your machine is likely to be technically obsolete before it dies from electronic or mechanical problems.