This should help, it worked for me (took me weeks to find out how to fix the problem).
I worked in in office Last year that had about 40 work stations consisting of Acad R14, LT 2000, Arch Desktop 3.0 & 3.3 - and me with Acad 2004. I had the same problem - But only with people running Arch desktop version all the others were fine. So here's what happens (if you know some of this then please just walk through it with me) When ever you open a Dwg in Arch Desktop (this also applies to Land develper and Mech desktop) - unlike Plain Acad, it will load "AEC Enablers" (aka "features" reserved to that version of Acad) to the dwg at start up. once the AEC Enablers are loaded and the dwg is saved they are then embeded in the dwg (you may notice that all to annoying "Proxy objects dialog box" warning while it can be suppressed it is a sign that there are enablers embedded in the dwg.
Acad 2004 will automatically update all the AEC enablers it finds in these dwgs. But whats happening in 2004 happens without your control and cannot be disabled.
There are default settings that in full versions of Acad that will set Autocad to (if you have an active internet connection) down load the AEC Enablers and update Acad. You can also down load a 2004 fix to update all the AEC enablers to 2004 version.
This is a NO NO, If you were to update the AEC Enablers in these dwgs then only you with 2004 or someone with ADT 2004, or what ever Acad verticle your using would be able to open it.
So here is the other option I had, either everybody upgrade or try one of the following fixes:
1) If you Do Wblock and choose "Entire DWG" on any of the offending dwgs Acad will strip out the AEC Dictionaries (the stuff the Enables embed in the dwgs). And this will fix the problem. (But let me guess what you saying "I can't be bothered with doing that eveytime I work on a dwg") Bingo niether can I. So I did the next best thing.
this is option 2
2) I use a program called Super Purge (I've been useing it for years) and found out from talking to the guy that makes Smart purger that the AEC Dictionaries can be safetly removed and then those nasty errors will go away. - Acad's Purge utiltiy will not remove the AEC Dicts.
I don't know about Samrt purger, But Super Purge will allow me to make a "Start up" purge list (which in my case Purges AEC Dictionaries only).
So eveytime I opened up a DWG Super purge would scan the dwg and Automatcally remove the AEC Dictionaries. This is the only way I have found to fix the problem you mention I have to assume your co workers are using an Acad verticle not Plain Acad, because I work no with a guy now who uses plain Acad 2002 and I don't purge the Dwgs anymore and he hasn't complained yet. Good luck HTH
BTW I think Super purge costs about $15-$20.00
But be warned If your co workers use ADT's advanced feature (like setting schedules, Walls, etc) then removing AEC Dicts will screw up those things and turn them back into basic Acad objects (lines, text, etc).
In my Last firm my co-workers didn't - they didn't even know how to.