I'd be surprised if the OS is the problem. I had no trouble installing Eudora 5.2 (the full version with compulsory ads) on XP and that was more than a year ago. Your defences are probably too strong.

The ads are not a problem in my view - they occupy only about 1.5 x 1.5". At first I got pop-up windows scolding me if I allowed other windos to obscure the ads, but even that doesn't happen now.