I've been there . . .
I also have a Dell XPS 400 and lost my mouse and keyboard, so I know the frustration you're having here. In my case the cause was spyware had attached itself to my USB driver to log my keystrokes -- when I attempted to remove the spyware it corrupted the usb drivers. If your problem is the same, then you don't need to reinstall the whole system, only the usb drivers. The problem is that you don't have 'human interface' with the computer, and the Dell XPS has no PS2 ports. Your BIOS usb drivers should run the keyboard and mouse though, until the corrupted windows drivers take over.
Can you access the root of the C:\ drive over a network? (No, didn't share the c root either, but if you did you could replace the drivers from another computer.) If not, you'll need a boot disk that loads USB drivers. Unfortunately, normal DOS boot disks can't read our SATA drives, so you'll need a boot disk that can load SATA drivers. Can you run the Dell diagnostic CD -- it should boot from it and check to make sure your hardware is okay. I wound up downloading a temporary copy of Active@ Boot Disk from NTFS.com -- I had another computer, so I could burn this to a CD and boot from it. This allowed me to get enough control to replace my USB drivers, and now I'm fine. Well, my computer is, anyway. The software comes in full functional limited time mode, so if you're budget constrained this won't cost you anything, although I recommend buying it ($80) if it works for you. You can try other recover boot disks -- SPOTMAU works on the XPS400, but I coundn't get the UBCD (Ultimate Boot CD) or UBCD for windows to work on the XPS.
Your USB drivers will be in C:\Windows\Sytem32 and C:\Windows\Sytem32\drivers. I had four drivers corrupted (I could only tell by reading the properties on the drivers -- the original Microsoft drivers had all the fields filled in, the corrupted ones were blank). I don't know which USB files are critical or not, so I basically replaced everything starting with USB. I can't remember which ones were corrupted, but I think they were usbport.sys, usbhub.sys,and usbuhci.sys, and one other. Hope this helps. Hope the novel doesn't exceed TechSpot's bandwidth. Good luck, and remember -- Computers are here to help us. No, really.