Nvidia usually makes its own drivers for Linux distributions. These kinds of drivers are known as "proprietary drivers", which means that their code is made and developed by Nvidia developers. In Ubuntu, in case that something goes wrong (including a driver), developers (from anywhere in the world) start working (on open source code) immediately on a fix so that it is available as soon as possible in the Update Manager. But with Nvidia, the situation is different. Because it's a proprietary driver, Ubuntu's developers doesn't have access to the code (because it isn't a public or open source code), so they can't improve or fix it. Any user using a Nvidia graphic card under Ubuntu has to wait for Nvidia's developers in case that it appears a problem with the driver.
So here's my problem. After I activated Nvidia's driver (version 173), it asked for a reboot. Until then, everything worked fine. But after reboot...it happened...I don't know what. I can't even explain. Under the VGA port, the image was highly affected. I could hardly read something on the screen (the colors and the image were totally misplaced). The monitor (Samsung T220) was continuously warning me that the image is displayed in "not optimum mode", and even after I got into driver's settings and changed them from auto to 1680 x 1050 with 60 Hz, the situation remained the same. The image on the monitor was starting to color white at every 10 seconds, and I was afraid that this can burn the pixels or even broke the entire monitor.
On DVI, the situation was completely different. The image looked pretty OK, however, the monitor was still warning me that it is in "not optimum mode" and it was restarting at every 1-3 seconds. This happens both with driver version 173 which is recommended, and with version 96 to which I have access.
In now stuck on a desktop without any accelerated video driver, but at least, this works (even if it is quite slow). The only thing I can do is to hope and wait for Nvidia to update their stupid driver for Ubuntu 10.04. I don't know why, but I think that Nvidia won't change something here...I hope I'm wrong. I know that there are some open-source drivers for Nvidia called "Nouveau" but I observed that they are only for GeForce6 and above (mine is GeForce 5).
So, if you installed one of these drivers with problems, try to enter into "hardware drivers" to remove it, and then reboot. In this way, you will be able to use your computer.
If you know how to fix this, please let me know via a comment.