Windows defender definitions update problem

I downloaded MS Windows Defender and it downloaded and started fine. It shows as the little castle in the system tray with a yellow ! and runs although it says the definitions haven't been updated in 203 days. When I select the update definitions options it shows that it is going out to the site to download updates but after a few minutes returns a message that it was unable to update the definitions. I am guessing it is a firewall issue but thought I would see if anyone else had this problem before making changes.

I did run all the steps from the tutorial on removing Malware and also the tutorial on protecting yourself from malware. I had no issues with the Spybots S&D or Adaware SE except some cookies. The online bitdefender and Panda scans came back clean. I ran my HJT scan through the online analyzer and it only returned a couple of unknowns that I recognized so I think I am pretty clean there.

I run NAV corporate edition as it is supplied by work and required by policy in order to use our corporate VPN.

Any thoughts are appreciated.

I am running XP service pack 2 on a Dell laptop. It is at home so I don't have specifics.

Thanks..AlasKen

 

Just to avoid confusion I should note that this problem is on a different computer than the one in my previous post concerning security hijack malware. After spending the time on that computer I am giving all the home computers a check up. Thanks in advance..AlasKen

 

Also If you use microsoft Update instead of windows update it will include the definitions file in its updates. (I think...mine does anyway)

 

After following your advice it still wouldn't let me update via Windows Defender. I went to Update.microsoft.com and it recognized that the definitions needed updating and updated them.

Can you explain the difference between between Microsoft update and windows update? Can I set it up so Microsoft update is automatic? Thanks for your help.