I easily get over 50 hours battery life from my Desire. I simply have GPS/Wi-Fi turned off when not using them, and also mobile data (which I turn on whenever I need it).
If I have mobile data turned on all the time then I get about 40-48 hours, depending on usage.
I would say if your Desire is dying, you are probably having everything updating far too regularly, don't forget GPS will kill any phone's battery dead if you leave it on as well.
I'd also recommend installing "Sweet Dreams" app, so you can have it automatically turn off data/wifi ect. when you're asleep.