Instructions on how to update security essentials more frequently than every 24 hours - typically MS seem to do about 3 updates every 24 hours or so
Follow the guide to tweak the registry to change the Microsoft Security Essentials signature update interval:
Run Registry Editor (RegEdit).
Navigate to the following registry key:
HKEY_LOCAL_MACHINE/Software/Microsoft/Microsoft Antimalware/Signature Updates
In the right pane, take ownership and grant full control permissions on a registry key, a REG_DWORD value named SignatureUpdateInterval.
Double click on the SignatureUpdateInterval. By default, its value is 24 (in Decimal) or 18 (in Hexadecimal). Change its value to any number between 1 and 24 (both inclusive) in Decimal.
The numerical value determines how frequent and how fast (in hours) Microsoft Security Essentials will check for updates and update the virus and spyware definitions.
Restart the computer.