Trial Editions can only be used for a limited amount of time, usually 90 to 180 days. After that time period has expired, you cannot use the product again without a valid product key.
To obtain a product key
Purchase a copy of Visual Studio 2005 Professional Edition at a retail location. The product key is listed on the sleeve of the DVD or CD.
—or—
Order a Visual Studio 2005 Professional Edition product key online at http://msdn.microsoft.com/vstudio/howtobuy/. The product key is sent to you in an e-mail message.
To upgrade Visual Studio 2005 Trial Edition
On the Start menu, click Control Panel.
In Control Panel, open Add or Remove Programs.
Select Microsoft Visual Studio 2005 Professional and then click Change/Remove.
On the Visual Studio 2005 Maintenance screen, enter the product key in the section Upgrade to Visual Studio 2005 Professional and then click Upgrade.