Installing Vista - Almost completed

Okay, so I have an Dell XPS 710 and received my Vista upgrade dvd yesterday and started the upgrade (kept old files etc.). The installation went fine until the "Completing installation" reached 57% and the system rebooted. Then I got the options: Windows Vista, Previous Windows systems or Rollback to a previous operative system (or something like that). If I dont choose either within 2 -3 seconds the machine picks the one that is highlighted (which naturally is Vista). So far so good. But then it sends me into a "Boot error"-menu which tells me to either start in different safe modes or in normal mode. Once again if I dont choose any, the highlighted (wich is "normal mode" ) gets chosen automatically within 30 seconds. Then it seems to start but when the "green-scroll" with black background (the one displaying everytime the computer starts - blue in XP) shows it freezes for like 5 seconds and the system reboots and the process happens all over again and again until I turn the computer off.

 

I tried to "rollback" the system back to XP and reinstall Vista, but the same thing happens. I also talked to technical-support-service which told me it was not a hardware problem so he couldnt help me out. I now run my system in XP, but I want to upgrade to Vista, so if any of you know what may cause this and also what to do I would appreciate it and give it one more shot. Thank you in advance.