Apparently sometimes Setup Wizard is unable to attatch all ActiveX controls all by itself. The same thing happened to me. If you search for the ActiveX file on problamatic clients, you wouldn't find it. So you need to copy the file manually (in your case comdlg32.dll) in c:\windows\system subfolder (c:\winnt\system for 2000/nt). Then run REGSVR32 COMDLG32.DLL .
About those clients that have no problem with ActiveX controls, they must have the full version of Access installed on their machines.
Hoping it solves your problem, even thou you should be fine after 4 months!!!