My Dell Inspiron 1720 is stuck at the loading Vista screen. How do I fix this?

when you boot again try last known good configiguration located in the advance start up. (press F8) after boot. also there is repair your computer but last known good congig is for start up problems.

There are a bunch of things to try, the other answers are a few, but more importantly you are interested in saving your existing data. So before you go much further, remove your drive and put it into an adapter (they're cheap, like under $20) that allows you to access the drive through USB. Then copy at least all your important personal stuff off to another system (don't bother with copying Windows or your applications. It won't work anyway). Once you have that done, go ahead and replace the drive and try to boot to safe mode (your repair mode) as described by others. If you uncomfortable with your ability to make good decisions about what to kill in safe mode, don't do anything rash, get help. Also, make good notes about each thing you try, so if you cause more problems you know how you got there. You can also try working from a bootable CD, either Windows (possibly got one with your system) or a Linux version. At least that will let you look around the system, but again, go for help. But do the backup first!

Try booting into safe mode then see what new installed programs would cause this problem.