Posting more details about your setup will help. When diagnosing these types of problems I try the following steps, one at a time:
1. Replace memory modules with known good ones. Make sure the memory speed matches your system.
2. If you're over-clocking the processor, that could be your problem. Return the BIOS to the default settings.
3. Check out your video drivers website. nVidia has some drivers that are certified for specific applications; you may be running into a problem if you're using the latest driver.
4. Defragment your hard drives.

Good luck, these types of problems can be very frustrating. Make small changes, one at a time. Keep a log book of what you've tried and the outcomes. Take a scientific approach to it.

Hope this helps!

In the case of the nVidia drivers (and not to belabor this), driver version 162.65 was the last one to support full-screen playback on a second monitor. The next release, version 178.26, did not. I believe the latest is v. 181.xx, which based on the answer I got from nVidia (posted on another thread) also will not.

They also noted that all Vista drivers do not support this. Oh well.

Another notion that I have toyed with, since my system is for general usage and has lots of non editing apps onboard, is to put the C: drive in my VanTec removable drive tray.
Vantec EZ Swap MRK-200ST-BK*C SATA Removable Hard Drive Mobile Rack Tray with LCD Display: MRK-200ST-BK*C (Black, Retail, Cartridge only)
It takes literally 5 sec. to switch out a SATA drive tray before booting up.
Then I could put a new C: in the tray, install XP (or whatever), install audio & video drivers, CS3, and absolutely NOTHING ELSE. If I were going to have an editing session I would slip in the C: drive for Editing & boot up, the rest of the time I could leave the General Usage C: drive in the tray. I'm imagining that it would be like having two seperate computers for the price of one. I've never done anything like this, so there may be some drawback I'm unaware of, but it might be a cheap, clean solution. I'm thinking that it would liberate a lot of system resources- and that seems to be the main problem I'm currently having with CS3.
Anyone ever done this????

Yeah, I use a trayless drive swapping solution from Kingwin on many of our editing systems. I have XP32 on one drive and XP64 on the other. Same basic idea though and works great besides the software licensing issues.

I installed CS3 as well as CS4 so I could use Prospect.

I am still getting the same crashes with CS3, so more memory and Vista 64 did not solve the problem. I could have some corrupt files that are causing the crashes, but I can't find them.

Perhaps this is to be expected, as CS3 isn't officially supported on any 64-bit OS and no one said CS3 could use more memory in such a setup, unless I missed something. But hope always holds out...

I think the difference I am most curious about is with CS4 -- with or without Cineform -- does 32-bit vs. 64-bit with more RAM make a difference? And based on Ray's experience, maybe not.

CS3 apps (AE and PPro) will have access to more Ram on 64bit OS. (4GB instead of 2GB)

Then I stand corrected. My understanding was CS3 could never use more than 2 gigs.