Just answered (or rather tried to help) on this matter, below i have cut and pasted what i typed.
Is this just when opening AutoCAD or when you are trying to open a specific drawing?
I have had man errors over the time of using AutoCAD (should start to keep a list)
In the past it has been one fo several issues.
1. Graphics compatability - solution drivers (in most cases)
2. Corupt dwg - open it on the machine that can (usually the machien the dwg originated from) then type Audit in the comand line and then Y to fix errors. Save as and choose a different name then try opening it on yours.
3. Virus and or spyware - recently had a problem with this, used adaware, spybot (also known as search and destroy). These found alot but did not help the fatel error. Eventually using Hijack This and posting on a PC proffesional forum sorted it out. Just had this same problme last night on a laptop with 3DSmax.
3. is the most unlikely.
Also a quick google search has thrown this up
Please check the following:
Have you applied the Service Pack 1 for AutoCAD 2004?
Do you have other applications running or installed lately (Cisco security agent, VPN, antivirus, etc.)?
Is your disk structure correct? Perform CHKDSK on your disk and check the TEMP folders.
Any intermittent software problems can be also caused by hardware failures.
Let me know hwo you get on or if you need any links.
Cheers.....Q