Computer freezes just after Windows is started

Hi everyone,

For a while, I have freeze problem with my 6-months-old PC with Windows 7 Pro 64-bit. It happens just after I typed account password on log on screen and it is a hard hang. The mouse and keyboard, everything is unresponsive. Sometimes, it freezes even before the desktop is seen (it happens when there is the "Welcome" message). About half of time, NVidia driver stops responding and is recovered before freeze. I tried all versions of NVidia in case of a driver problem, didn't solve the problem.

At first, I thought that it was a driver related problem with Windows, because when I started Ubuntu directly from CD ("Try Ubuntu" thing), there was no freeze (probably since there is no nvidia driver installed, but the display was just fine, full resolution, I could watch some movies, etc.).

However, then I installed Ubuntu on one of the partitions (This time it installed NVidia drivers with version 270.xx), and there is the same freeze problem just after I log on to Ubuntu. So, I think this is either a NVidia driver incompatibility with drivers of other hardware (so that can be in Ubuntu, too), or a directly hardware issue.

I checked RAMs both manually (plugging every one of them with different slots) and with Memtest with 9 cycles. I also checked HDD. USB ports were also checked. I tried changing some parameters in BIOS, then setting default again. Nothing positive happened.
Then I tried an ATI 5750 video card on this PC and there is no freeze problem, at least in the first few hours, I didn't have a chance to try longer.

Thus, I suspect on three things: 1.My video card is defected. 2.My video card is incompatible with other hardware. 3.My PSU is just insufficient for my hardware.

What can you say or suggest about the problem or my suspicions? Do you know any problems about my video card or other hardware? Do you think the PSU is sufficient for my system?

P.S: I don't know if my system specifications can be seen (I entered them when signing up) under the post, but if they can't be seen, I will give them in the next post.

Have you tried newer video drivers? Nvidia is up to 280.26.

Yes, I tried the newest driver version as well as older versions. I had the same problems in all cases.

The same problem in both Windows and Ubuntu makes me think of a hardware problem in video card, like a malfunction. A hardware conflict between video card and other hardwares, or a driver conflict between hardwares might be other causes.

Also, can PSU insufficiency cause freeze problems, or is the PSU just fine for this system? That is, can I rule PSU out of possible causes?

Minimizing the cause to a single hardware is important for me, because I bought different parts from different places. I want to contact with the right store if it comes to the repair/replacement.