This link will bring you to an article that explains how to determine if you have the latest definition for Windows AntiSpyware (Beta).

http://support.microsoft.com/default.aspx?...kb;en-us;892519

Article Not Available Anyore

Try this from the above link "basic search of the knowledge base" for Windows Defender or you can use the above link and do your own search of the knowledge base (use the box just below "products" and type in your request.
http://support.microsoft.com/search/defaul...fender&adv=
Good luck.

The link provided by mlurp works. Here the latest definition for Windows AntiSpyware (Beta) can be read. Thanks mlurp.

The last update for Windows AntiSpyware (Beta) was on 12 August 2005. Its version number is 5745.

You can find definition update information at our security portal. Currently signature updates go up on Windows Update and Microsoft Update on Tuesdays and Thursdays. You can download updates through Automatic Updates, the update feature in Windows Defender, or via the manual update links provided at the portal.

Here's some links that may be useful:

Microsoft Malware Protection Center - Security Portal
http://www.microsoft.com/security/portal

Microsoft Knowledge Base Article 923159
How to manually download the latest definition updates for Windows Defender
http://support.microsoft.com/kb/923159/

Hope this helps,

Aaron