No computer exists that is able to run IE7 but not IE8. And because IE7 came out way past the "optimized for IE" age, I doubt there is software around that requires 7 and doesn't work with 8.
This is in contrast to 6 which is what IE was stuck in for years, so software began depending on that.
If you could upgrade from 6 to 7, then you can upgrade from 7 to 8. Even more so considering that IE8 has an IE7 compatibility mode built in that can be enabled without having to touch the site itself (by setting the HTTP header in the server).
The same is true for local software embedding the IE control, but there you need to set some registry flag to do it.
There is no excuse for sticking with 7 and 8 is so much better to develop for. It's 2014. I would stop caring about 7. People still using 7 are used to sites not quite looking right anyways (as the majority of sites stopped caring) and in all likelyhood, they might not even notice the odd rendering glitch.