Nividia driver update

I am writing about some unusual behavior demonstrated by my XPS M1330 PC.  I obtained the PC in used but almost like new condition.  Its operating system is Windows Vista Home Premium with all updates and its BIOS is also up-to-date (A15 at this time).  The PC contains a Core 2 Duo T9300 2.5MHz CPU with L2 6MB cache, a 320GB hard drive, 4GB RAM, a SigmaTel sound card with Soundblaster Audigy software (I may not be entirely correct on this, but this is as best as I can verify), and a NVIDIA GeForce 8400M GS video card.  As briefly as I can, about 3 weeks ago, I had installed a video card driver update from the Nvidia website after verifying that the driver was intended for my particular video card and PC.  The PC has worked flawlessly until three days ago, my PC began freezing up and showing the dreaded blue screen with the following message:

***Hardware Malfunction
Call your hardware vendor for support
NMI: Parity check / Memory Parity Error
*** The system has halted***

This began occurring every 5-15 minutes whenever I played a video or played my combat aviation game.  I suspected that the Nvidia driver was the culprit and deleted it a day later.  I continued to receive this message, so I searched the Dell support site to see if there were any updated video drivers for my specific video card.  I found one driver, R200273, dated 10-23-2008 and downloaded it.  After installing the driver and re-booting the PC twice two days ago, the PC appeared to work flawlessly again until today, when the PC started to freeze and unfreeze repeatedly.  At the end of each cycle, a message balloon indicating that the driver nvlddmkm had stopped working and successfully recovered.  A Google search revealed multiple forum topics about this driver, and they suggested that the Nvidia video drivers were malfunctioning.  After ascertaining that the Dell website did not contain any newer video driver updates than the 10-23-2008 one I already installed, I visited the Nvidia website and found a newer driver, 185.85, dated 5-7-2009, and downloaded the file to my downloads folder, then ran and installed it.  The usual installation "shield" appeared, but after running a few minutes, the screen went blank, then appeared to be covered with multiple colorful fragments, like a multi-color tapestry.  I left this alone for 5-7 minutes in case this was simply part of the installation process.  However, when nothing changed, I tried moving the mouse indicator, using the "escape" function, and tried opening the task manager with the ctrl-alt-delete sequence, all to no avail-->the screen stayed frozen with the multiple colorful fragments.  Since I could think of no other method, I resorted to turning off the PC with the on-off switch.  I re-booted the PC in normal mode and interestingly it seems to be working again.  I saw the message balloon indicating again that the driver nvlddmkm had stopped working and successfully recovered one more time, but for the past 60-90 minutes, the PC has been working apparently flawlessly, with no further message balloons.  I watched videos and played the aviation game without problem.  I also checked the folder C:\\nvidia\winvista and the updated driver 185.85 from the Nvidia website appears to be installed.

My question after this long preamble is if the rainbow screen that ended the installation of the 185.85 Nvidia video driver indicated anything amiss?  I'm glad the PC appears to be working normally again and perhaps I should be happy enough with this result, but the rainbow screen and having to turn off the PC manually without a proper shut-down was disconcerting and I just want to be sure as much as possible-I know almost nothing is 100% certain--that I don't have to worry about this aspect any longer.

Thanks much for your attention and any advice/opinions.

PG