I've gotten a windows application to read an excel file with the following 
code:

        string ExcelConnStr = @"Driver={Microsoft Excel Driver 
(*.xls)};DBQ=\\Server\Spreadsheets\Test.xls";
        OdbcConnection ExcelConn = new OdbcConnection(ExcelConnStr);
        ExcelConn.Open();
        ExcelConn.Dispose();

However, when I use the same code in a CLR Stored procedure with an external 
access assembly on a trusted database, I get the following error:

System.Data.Odbc.OdbcException: ERROR [HY001] [Microsoft][ODBC Excel Driver] 
System resource exceeded.
ERROR [IM006] [Microsoft][ODBC Driver Manager] Driver's SQLSetConnectAttr 
failed
ERROR [HY001] [Microsoft][ODBC Excel Driver] System resource exceeded.
System.Data.Odbc.OdbcException: 
   at System.Data.Odbc.OdbcConnection.HandleError(OdbcHandle hrHandle, 
RetCode retcode)
   at System.Data.Odbc.OdbcConnectionHandle..ctor(OdbcConnection connection, 
OdbcConnectionString constr, OdbcEnvironmentHandle environmentHandle)
   at 
System.Data.Odbc.OdbcConnectionFactory.CreateConnection(DbConnectionOptions 
options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection 
owningObject)
   at 
System.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnectionPoolGroup poolGroup)
   at 
System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection 
owningConnection)
   at 
System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection 
outerConnection, DbConnectionFactory connectionFactory)
   at System.Data.Odbc.OdbcConnection.Open()
   at StoredProcedures.usp_ImportExcel()
.

Any help is appreciated ...

This is an interesting problem. nine times out of ten, the problem actually lies with the SQL Server system user not having the correct permission for the network share. The driver never seems to give the right error information, for some reason. I realise it could be something else, but it is so easy to check that SQL Server has the right permission.