"I'm using the ATI drivers that windows update automatically downloaded"
Microsoft rarely gets drivers right. Now go get the current drivers for your machine from the makers of what part you want a driver for.
I write this because our Dimension C521 is having problems going into sleep or standby mode, and because it's having what seems to be a video issue.
The first problem is that when the computer goes into standby, it will wake up after only a few seconds. It goes to sleep, alright; the hard drive will power down, the light in the power button will start flashing, but then it just comes right back on.
The second problem is that, once the computer has been on for a few days, the display will stop working. What I mean by this is that the computer is still on, the hard drive activity light still flashes, but nothing is displayed on the screen. I suspect it to be a video card issue because: a) the monitor works fine, and b) the power light on the monitor will turn yellow. The only thing that can be done in this situation is to hold the power button to shut down the computer. Could it be the video card drivers, or is it a possible hardware problem with the card itself? I'm using the ATI drivers that windows update automatically downloaded; they're dated 6/2/2008.
Here are the computer specs:
AMD Athlon X2 3800+ (AM2 socket)
4 x 512MB DDR2 667
250GB SATA hard drive
128MB ATI Radeon X1300
Dell motherboard (nForce 430 chipset)
Vista home premium 32-bit SP1 with latest updates (a pretty fresh install; I reformatted last November)
Antivirus is Norton 360 2008 (or whatever the latest version is)
The computer is about 2 years old.