Did you try to start in diagnostic startup? Start --> Run --> msconfig. Choose diagnostic startup and see if you can boot the computer. If so, the problem may be with third party software and not device drivers.

If the problem just started, check out system restore and see what's been installed lately. A new device of some sort? Otherwise, download the latest drivers for the components from dell and other manufacturers. Burn them to a CD if you can't use your flash drive. Then start updating the drivers. I prefer to uninstall the device/driver and install rather than just update the driver. Don't forget the update the BIOS if needed.

@ShagDevil: A repair install or sfc didn't fix that problem?