How To Alter A Table In A Trigger?

Hi,everyone.

I have a problem with SQL SERVER 2005, described as follows,

(1) To create two tables MASTERINFO and PRODUCT,

----TABLE MASTERINFO-----
CREATE TABLE MASTERINFO
(
ID CHAR(2),
FIELDNAME VARCHAR(50),
FIELDTYPE VARCHAR(50),
);

Table MASTERINFO with following records,

MASTERINFO('1','PRODUCTNAME','VARCHAR(50)');
MASTERINFO('2','PRODUCTADD','VARCHAR(50)');
MASTERINFO('3','PRODUCTEXP','VARCHAR(50)');

------TABLE PRODUCT-----
CREATE TABLE PRODUCT
(
ID CHAR(5),
PRODUCTNAME VARCHAR(50),
PRODUCTADD VARCHAR(50),
PRODUCTEXP VARCHAR(50),
);

In our project, field name and field data type of fields PRODUCTNAME,PRODUCTADD and PRODUCTEXP in the table PRODUCT are changed with values of fields FIELDNAME and FIELDTYPE in the table MASTERINFO.That is to say, when using the following UPDATE statement,

UPDATE MASTERINFO SET FIELDNAME='PRODUCTNAME1',FIELDTYPE='VARCHAR(60)' WHEN ID='1';

We hope that field name of field PRODUCTNAME is automatically changed into PRODUCTNAME1 and data type of its is changed into 'VARCHAR(60) in the table PRODUCT.

I want to use a trigger to realize, but I failed for ALTER TABLE statement can not be included in the trigger. How can I do?

Please give me some advice. Thank you in advance.