If you have been keeping Windows and your anti-virus updated, it is doubtful that you have Blaster. More likely malware has planted that alert on your computer in an attempt to scare you into to purchasing removal software.

You can run the free removal tool available here:

http://www.symantec.com/security_response/writeup.jsp?docid=2003-081119-5051-99

• Make sure a checkmark is placed next to Update Malwarebytes' Anti-Malware and Launch Malwarebytes' Anti-Malware, then click Finish.
• If an update is found, it will download and install the latest version.
• Once the program has loaded, select "Perform Quick Scan", then click Scan.
• The scan may take some time to finish,so please be patient.
• When the scan is complete, click OK, then Show Results to view the results.
• Make sure that everything is checked, and click Remove Selected.
• When disinfection is completed, a log will open in Notepad and you may be prompted to Restart.(See Extra Note)
• The log is automatically saved by MBAM and can be viewed by clicking the Logs tab in MBAM.
• Copy and Paste the entire report into a New Message on the HijackThis Board. Also include a fresh HijackThis log. Instructions for downloading HijackThis are at the top of that forum.
  1. Just click the New Message button in the HijackThis forum here: http://www.dellcommunity.com/supportforums/board?board.id=si_hijack
  to start your own thread requesting assistance for a follow-up check to be sure the malware is gone.
  2. In the Message Body window that opens, simply Right-Click and select Paste.
  
  

If the worm disappeared it is hidden in your pc and is still there.  

Follow the directions in this post and post your results in this forum.