cmaudio.sys blue screen

What model PC? and what Operating System?  We cant give you assistance without that.  All we can do is offer general comments.

That is part of the sound drivers.  You can check the Dell downloads for your specific model and see if there is an updated driver.  http://support.dell.com/

Dell Dimension 8400, Pentium 4 with Windows XP Professional Service Pack 3

I checked the C-Media site and the Dell site but didn't find anything obvious.

HERE is the Dell Analog Devices integrated audio driver.  Reinstall that and see if it fixed it.

The install failed. I got this rather useless message:

"The audio driver files do not support your computer hardware.

Note: If you uninstalled audio software without restarting your PC, restart now, then run this setup again."

It sounds like you have a PCI sound card, such as a SoundBlaster model rather than the Integrated (on the motherboard) audio. 

Please confirm what sound device you have.

Here is the report from System Information, Sound Device:

Name Unimodem Half-Duplex Audio Device
Manufacturer Microsoft
Status OK
PNP Device ID MODEMWAVE\0\{167B4D0D-8833-43FB-9198-37FD06FB174B}
Driver c:\windows\system32\drivers\modemcsa.sys (5.1.2600.0 (xpclient.010817-1148), 15.75 KB (16,128 bytes), 10/25/2004 11:13 AM)
 
Name CMI8738/C3DX PCI Audio Device
Manufacturer C-Media
Status OK
PNP Device ID PCI\VEN_13F6&DEV_0111&SUBSYS_011113F6&REV_10\4&10416D21&0&10F0
I/O Port 0x0000CC00-0x0000CCFF
IRQ Channel IRQ 18
Driver c:\windows\system32\drivers\cmaudio.sys (5.12.01.0643, 368.51 KB (377,358 bytes), 11/18/2002 2:51 PM)

The microsoft device should be my webcam...

HERE is a site to download the C-Media drivers. 

The Unimodem is part of your dial-up data modem, not your sound device.  There is nothing in the Dell downloads that references C-Media and the only thing I can think of is that your dial-up modem card contains this device.  The date on the driver (2002) suggests it's an old device as the 8400 is much newer than that.

Your sound device will either be the Analog Devices ADI198X integrated audio or the optional SoundBlaster Audigy2 ZS PCI sound card.  Those are the only sound items listed in the Dell downloads for the 8400.