Window replacement, what do you do?

[Jim K.]

I have 18 windows to replace in the next few days. To keep my interest up and to keep myself accountable, I'm starting this thread. I am a self-managing DIY landlord specializing in 70-100 year old SFR properties in my city and its near suburbs. How do you handle window problems in your properties? Do you contract it out to a specialized window-replacement company, hire a handyman, or DIY it? Why? What kind of properties have you changed windows for? Do you prefer to fix or improve the existing windows?