I have installed the Informix-CLI SDK 2.40 (driver ODBC rev. 3.31) both in Windows 98se and NT 4 sp4, and I've encountered a problem with Visual Studio Data View when try to see tables from an Informix DB running on an AIX 4.3 host. I have problems with numeric data; the view is incorrect (0 when it must be 1). An older version of CLI SDK works correctly.
I've seen several reports of strange behavior in 3.31. If an earlier version works better, stay with it. Give the latest version a few releases to work all the kinks out.