Try this workaround:
1. Remove the current signature file. To do this, click Start, click Run, type
Msiexec /x {A5CC2A09-E9D3-49EC-923D-03874BBD4C2C}
and then click OK.
2. Open Windows Defender. To do this, click Start, click Programs,
and then click Windows Defender.
3. Check for new definitions. To do this, click the Help options arrow next
to the Windows Defender Help icon, click About Windows Defender, and
then click Check for Updates.
"Windows Defender definitions haven't been updated"
http://support.microsoft.com/Default.aspx?kbid=915105
--
Carey Frisch
Microsoft MVP
Windows - Shell/User
Microsoft Community Newsgroups
news://msnews.microsoft.com/
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"Enoch Root" wrote:
| Glad I'm not the only one. I've been getting the same thing. It's also been
| trying to update automatically for several days and failed. If you try via
| Windows Update, critical updates then it downloads but also fails to install.
| It says the problem occurs after you uninstall and try to reinstall Windows
| Defender, but I have never done either, and I upgraded from Microsoft
| Antispyware, so have been using it for a year. There is definately a bug at
| microsoft's end.