You have 180 days to evaluate.  You can check number of days till expiration:   open your SQL Server Management Studio and widen first line in Help — About box.

What if your trial is about to expire, and you need just a few more days to evaluate?

You can switch to a free Express edition, or you can follow this cumbersome procedure.

=01= Un-Install SQL Server 2008

Drop all your custom databases
Run your Maintenance Plan one last time
Save all your maintenance plans (this step was not tested)
Go to Control Panel – Programs and Features
Uninstall Microsoft SQL Server 2008 – Remove
Check Rules – Next
Select Features – You have to delete everything including the instance
Pick  – Select All
Removal Rules – Next
Ready to Remove – Remove
Removal Progress – Next
Complete – Close
Make sure Microsoft SQL 2008 Server line is gone from Programs and Features

Question: Does the Server restart required? – No

Delete file   user.config from:
%UserProfile%\Local Settings\Application Data\Microsoft_Corporation\LandingPage.exe_StrongName_ryspccglaxmt4nhllj5z3thycltsvyyx\10.0.0.0

Question: What if file user.config is not there?
This file only appears during un-install procedure.

=02= Re-Install SQL Server 2008

Question: Can I reinstall SQL Server with the same exact instance name? – Yes

[Get ready to answer lots of question and pick lots of options]
Insert SQL Server 2008 disk and pick to run SETUP.EXE
Pick Installation (on the left) and New SQL Server stand-alone installation (on the right)
Setup Support Rules – OK
Setup Support Files – Install
Windows Support Rules – Note Windows Firewall warning – Next
Product Key – Pick your free edition (Enterprise or Express) or enter a Product Key – Next
License Terms – Accept the license terms – Next
Feature Selection – Pick only what you need:
– Database Engine Services
– Management Tools – Basic
– Management Tools – Complete – Next
Instance Configuration – Pick named instance and type your instance name exactly as before – Next
Disk Space Requirements – Next
Server Configuration – Pick NT AUTHORITY\NETWORK SERVICE for both
You would need SQL Server Agent for your Maintenance Plan, so change it to Automatic – Next
Database Engine Configuration – Pick Mixed Mode and enter a memorable password
Add Current user and your Admin group – Next
Error and Usage Reporting – Do not Send Anything – Next
Installation Rules – Next
Ready to Install – Install (12:12) (12:53)
Installation Progress – Next
Complete – Close

=03= Post Installation Tasks

Check expiration days in Help – About box – OK
(If required) Enable FILESTREAM feature immediately (before installing service pack)
Download latest Microsoft SQL Server 2008 Service Pack

Apply latest SQL Service Pack 2 (as of 2010/10/08)
Welcome – Next
License Terms – Next
Select Features – Next
Check Files in Use – Next
Ready to Update – Update
Update Progress – Next
Complete – Close

Attach all your custom Databases:
Databases – Attach – Add and navigate to your location – OK

=04= How to uninstall SQL Server 2008 SP 2

Check your SQL Server 2008 level before the action. Run this query:

SELECT
SERVERPROPERTY('Edition') AS 'Edition',
SERVERPROPERTY('ProductVersion') AS 'Product Version',
SERVERPROPERTY('ProductLevel') AS 'Product Level',
SERVERPROPERTY('ResourceLastUpdateDateTime') AS 'Last Update',
SERVERPROPERTY('ResourceVersion') AS 'Resource Version'
GO

Go to Control Panel – Program and Features and click on View Installed Updates.
Select Service Pack 2 for SQL Server 2008 and click Uninstall a program.
You might need to insert your original SQL installation CD.
Check your SQL Server 2008 level after the uninstall:

Happy SQL-ing…  These steps were tested on Windows 2008 Server.