Do you receive any error code? If not, you should take a look in Control
Panel > System and Maintenance > Administrative Tools > Event Viewer (run
as administrator), and see if you can dig up something. This problem could
also be caused by your firewall.
Alternatively, you can download and install definition updates manually
from Microsoft Malware Protection Center:
http://www.microsoft.com/security/portal/
Windows Help: Troubleshoot problems with installing updates:
http://windowshelp.microsoft.com/Windows/en-US/Help/26d54faf-bee6-4d4a-b5e7-2f08b53c2c4c1033.mspx
Charlie42