At lest for the stable version of Windows (ie. up to Windows 7), you can manually install definition updates following instruction at
http://support.microsoft.com/kb/923159/en-us (Defender) and
http://support.microsoft.com/kb/971606/en-us (Microsoft Security Essentials).
For Windows 8 the definition updates might not be available to manual download as it is a pre-release preview. You might try the above ones, but on your own risk. As the Windows 8 will be released to market (finalized), I guess that there will be also support
for manual updates as today for Windows XP/Vista/7.
PS: A Question type should fit your post better rather than Discussion.