Can you boot into Safe Mode?
The specifications I found for the XPS 420 shows that the dedicated graphics card which comes with that computer is a 256 Nvidia GeForce 8600GT, but you have listed the Nvidia GeForce 9800 GT. Did you change the graphics card?
If you did,do you still have the original which came with the computer?
There is the possibility that there is a corrupted driver, you could uninstall and then reinstall the drivers to see if this resolves the issue.
Before uninstalling the drivers you should download the new drivers from the NVIDA
website. As you can see in the image below there are two means of finding the correct drivers. Option 1: is the manual method, Option 2: is a automated scan which will find the correct drivers.
Don't open the drivers until after all of the drivers have been uninstalled and the computer has been restarted.
In the Start menu click on Control Panel, when it opens click on Programs and Features and wait until it is populized.
The programs are alphabetized, scroll down to the Ns where you will find the list of NVIDIA drivers similar to the one in the image below.
Important: Users of older motherboards with a Nvidia Chipset may find a ChipsetDriver listed, these are unreleated to the Display Drivers. If you uninstall a NVIDIA Chipset drivers, your system could become non functional and require a installation of the operating system.
Remove these drivers one at a time leaving the Graphics Driver the last to be uninstalled.
Place the mouse pointer over the driver you want to uninstall, this will highlight the choice. Right click on the driver, a window will open with the word Uninstall, click on this to uninstall the driver. After the driver is removed from the list go on to the next one.
Note: If while uninstalling the Nvidia Drivers the system asks for a RESTART after uninstalling a driver and it wont let you proceed when choosing "restart later", then its OK go ahead and restart and pick up in the Control Panel under Programs and Features where you left off.
Now that you are left only with Graphics Driver, uninstall it.
The system will ask for a RESTART, you should now do so.
Windows has as native graphics driver which will make it possible to continue with the installation of the new NVIDIA drivers. You should install the new drivers now.