Cars are paid for, in good shape with good tires. :)
I have been wanting to do the windows for years, but it's a low-priority item and we've always gotten along with them and prioritized toward other things. I think that windows would be nice for the energy savings (they are very leaky) and also the aesthetics. I think we'd enjoy them. So we'll probably do that and move it up the to-do list. However, I'll sleep on it for a couple weeks before deciding. I like investing too! We'll be in this house probably another 10 years at least. I don't think the energy savings will pay for the windows quickly at all... but I could be wrong.
Generally, we are just bulking out our savings. Our home could use new siding too, (50 year old aluminum) but I think we'll phase it out and do the windows first, siding later on. I can make a case to do the windows because they work, but they're not working well. I mean, we can live with them, but it's an opportunity to make a substantial improvement. Or, I could save the money and do windows+siding together later on.