Since in your other post you mentioned it was a new Studio, can assume that it's Vista and not XP.   Unlike XP, In Vista Defender is built into the operating system, so the standard uninstall/reinstall suggestions don't really work.

Don't have Vista or Defender myself, but Google has tons of hits for that error - apparently it's a common problem on all Vista PC's, not just Dell.   Looking through those results found a couple solutions that appear promising:

1)  Make sure the Defender service is started and automatic:  

Go to Control Panel then select System and Maintenance, then  Administrative Tools then finally select Services.
Scroll down to Windows Defender service and check if it's deactivated and/or stopped.
If it is,  double click the service to open it, then choose the option to start automatic on start up, then click on the Start service button.
Then click on Apply and Restart the computer, and see if Windows Defender now starts properly.

2) If you happen to have McAfee security software, there may be a conflict.  See here for instructions from McAfee:

http://service.mcafee.com/faqdocument.aspx?id=TS100610

Thank you,  I do have Vista and I do have McAfee.  I had already tried the instructions from McAfee and the error message still came up.  I tried your suggestion and when I rebooted the error message did not come up.  Maybe this will take care of the situation even though it is the opposite of the McAfee instructions that didn't work.

Thank you for your help.

Hope that did it.

Some other possible suggestions below, just in case.  And plenty of company, unfortunately.

http://social.answers.microsoft.com/Forums/en-US/vistasecurity/thread/574fb871-034c-4b42-b800-328af16a9177

go to check mycomputers security under malware click show me malware programs or something llike that click defender and turn on now

Don't know if anybody's still checking this topic or not since the last message was more than a month ago -- but I recently purchased a Studio desktop and have been experiencing the same problem. I did what any sane person would do and checked Google where I found the "plenty of company" you cited Alexandra, which raises a question: IF people have known for some time, like at least a couple of years, that Defender won't play nicely with McAfee or vice versa, why in the world would Dell offer a McAfee subscription for it???