MY YAHOO MESSENGER WON'T OPEN UP OR START??

Set a New Restore Point to prevent possible reinfection from an old one
Setting a new restore point AFTER cleaning your system will enable your computer to roll-back to a clean working state if needed.

Go to Start > Programs > Accessories > System Tools and click System Restore
Choose the radio button marked Create a Restore Point on the first screen then click Next Give the Restore Point a name then click Create.
The new restore point will be stamped with the current date and time. Keep a log of this so you can find it easily should you need to use System Restore.
Next go to Start > Run and type Cleanmgr
Click OK
Click the More Options Tab.
Click Clean Up in the System Restore section to remove all previous restore points except the newly created clean one.

You can find instructions on how to enable and re-enable system restore here:

Windows XP System Restore Guide or Windows Vista System Restore Guide
.
----------

Use the Secunia Software Inspector to check for out of date software.

Click Start Now
Check the box next to Enable thorough system inspection.
Click Start
Allow the scan to finish and scroll down to see if any updates are needed.
Update anything listed.

.
----------

Go to Microsoft Windows Update and get all critical updates.

----------

Here are some great FREE tools to help you keep from getting infected again. These tools use little or no resources so won't slow down your PC.

Concerned about Browser Security? Consider using Mozilla Firefox 3.0 with Adblock Plus and NoScript

To prevent unknown applications from being installed on your computer install WinPatrol 2008
* Using Winpatrol to protect your computer from malicious software

I suggest using SiteAdvisor. SiteAdvisor rates sites on business practices and spam. Safety ratings from McAfee SiteAdvisor are based on automated safety tests of Web sites.

SpywareBlaster - Secure your Internet Explorer to make it harder for ActiveX programs to run on your computer. Also stop certain cookies from being added to your computer when running Mozilla based browsers like Firefox.
* Using SpywareBlaster to protect your computer from Spyware and Malware
* If you don't know what ActiveX controls are, see here

Check out Keeping Yourself Safe On The Web for tips and free tools to help keep you safe in the future.

Also see Slow Computer? It May Not Be Malware for free cleaning/maintenance tools to help keep your computer running smooth.