Have you tried booting into Safe Mode to see if you can get that far? If you can you may have something that is starting up with Windows that is causing it to hang but is not starting up in Safe Mode. If that's the case you may be able to boot into Safe Mode and then go into your startup items and uncheck all the non essential items or do one at a time to try and narrow it down.
Using MSconfig to check your startup items.
You can also boot to Safe Mode and select Last Known Good Configuration to see if that will get you going.
Another option is to boot with your Windows DVD and try the repair options there.
There is also a possibility of having a hardware problem but that will require some advanced troubleshooting so you might want to take it somewhere for that.
Try booting into Safe Mode and choose Last Known Good Configuration. This will start your computer using the configuration that worked successfully last time you started your computer.
Or you can see if you can actually log into Windows from Safe Mode. If so then you can try a System Restore to put your computer back to a state before you started having this problem.
You can also edit your startup items using MSconfig to make sure that there is nothing set to boot with your computer that shouldn't be. This could be causing your system to hang.