ms-help://MS.MSDNQTR.2005APR.1033/enu_kboledb/oledb/251254.htm
SYMPTOMS
When trying to connect to an Access .mdb file, an ODBC error is returned stating “Disk or Network Error”. The Microsoft OLE DB Provider for the Microsoft Jet database engine may also return an “Unspecified error” (error code = 0x80004005) message.

The problem can occur only when the data source is opened under the Microsoft Internet Information Server (IIS) or a Microsoft Windows NT service but not under a logged-on user account.
CAUSE
Jet creates a temporary file when the engine is started. In doing so, it first checks the TMP environment variable and uses that path to define where the temporary file is created. If it doesn’t see a TMP environment variable, it looks for the TEMP environment variable. If TEMP is not defined, it then uses the Windows folder (/WINDOWS or /WINNT).

If TMP/TEMP is defined but points to a nonexistent folder, the error occurs.

Moreover, when opening the Access database through IIS or a Windows NT service, and when IIS or the service is running under the local system account, the TMP or TEMP system environment variable is used. When the Web services or Windows NT service are started using a user account, the TMP/TEMP user profile environment variables is used.
 
RESOLUTION
Define a TMP or TEMP environment variable and assign the variable to an existing folder. If you are accessing the Access database from IIS, be certain to check the system environment settings rather than environment variables of the logged-on user. Also ensure that the IUSER anonymous IIS account has access to the temporary folder.

You must restart the computer after changing any environment variables.
Environment variables are set under Windows NT 4.0 by going to the Control Panel and clicking System and then selecting the Environment tab.