XPS420 Freeze issue

Ideally, you want at least 4 gigs of memory when running a 64 bit version of Win 7.  That issue aside, I wonder if your tecnical difficulties have anyting to do with your Inel system device that communicates with your Nvidia graphics card?  I had one problem during installation of my full version of Win 7.  It looks like you have the same video card as I do.  See this link helps any:

http://en.community.dell.com/forums/p/19299208/19576523.aspx#19576523

It's just a shot in the dark (for your issue)  but you might want to read my post. I ever had any BSOD; however, I had problems getting Device Manager cleaned up until the correct system device driver was loaded.  Again, you have the exact same graphics card as myself and your system is similar (am XPS 420 versis mine which is an XPS 410. 

So...what can I do?

First, I would check your Device Manager.  Is it cleaned up or are there any conflicts?  If so, what are they?  If it's something you can identify, try to go to the vendor's website and download the latest drivers.  Sometimes, conflicts in Device Manager cannot be immediately identified and the unknown (needed driver) will throw itself under 'Other Devices'.  That's when it is more difficult (and what happened to me) when I upgraded from Vista to Win 7.   I never got a BSOD but new there was an issue in Device Manager and it was determined to be the Intel system device that communicates with my video card.  I only mentioned that to you because I saw you have the exact same video card and your dell model is very similar to mine. However, again, it may have nothing to do with your blue screens.  However, you may want to read my post about my experience in installing Win 7. 

Back to your situation, you could also check event viewer (access that through your search box).  Event Viewer will identify error logs on your system.  Sometimes that can be a clue as to what is causing a freeze or blue screen.  If you use this feature in windows, you only need to focus on the actual error logs, not the warning messages and so forth.  There are many things that can do wrong so unfortunately you are going to have to do some investigation.  There's no single solution to fit everyone in your situation.  I wished I could tell you more.

However, since you are now using a 64 bit operating system, I would deffinately upgrade to at least 4 GB of ram.  What you have is quite low for a 64 bit system.  If you cannot diagnose the problem, you may want to consider reinstalling the new operating system (hopefully after you've added more ram).  If you decide to add more ram, remember physical slots 1 and 3 are a pair and physical slots 2 and 4 are a pair.  So you want to keep like ram in the same pairs in order to keep dual channel mode in your memory and to ensure your computer is optimized. 

I wished I could give you a more definitive answer but again there's many variables in these situations. 

In the Event Viewer I've seen some errors about Dns client service...can it be the cause of freeze?

Maybe I've solved the issue: I've upgraded  the Network card driver of my Intel(R) 82566DC-2 Gigabit Network Connection and I haven't had anymore the issue.

But...it's better I wait some days to be sure

Thank you for your aid!

I hope the new driver for your network card has prevented more issues.

After two months,the problem reappeared:emotion-6:

It's a very strange problem because some times it doesn't freeze for hours;some times it freezes every few minutes.

What can I do?Please;help me:emotion-6:

Maybe I know the problem:the power supply. I changed it one month ago because I bought a new video card. It's a 500W power supply,but it isn'st of a good brand. 

How can I be sure that the problem is the power supply?