I suggest your colleagues were absolutely right and the database is corrupt. First of all let me get straight one thing...you said .mdb file, are you sure? As i remember 2003 version has .accdb extension. Hmph, maybe i am wrong. Well, at any rate i would recommend you a /decompile feature. Of course i hope you have a backup, if not - make it. It is an important part, because the recovery process might go in a wrong way...and you must be prepare for this. After getting a copy of your DB compact it. Next open a command prompt and enter something like this. It is one line, and include the quotes:
"c:\Program Files\Microsoft office\office\msaccess.exe" /decompile "c:\My Documents\MyDatabase.mdb"
Then Compact it again and try to conversion.
It might help. I hope.

If not, well...then try to search on this or maybe other forums. There are a lot of tech forums, where such issues discussed. Technet or Msdn - both are Microsoft official forums.