Saint Petersburg, FL
I don't have win8 yet but in win7 you could simply use task scheduler something like this....
Run: "C:\Program Files\Microsoft Security Essentials\MpCmdRun.exe" -Scan -ScanType 1
Start in: "C:\Program Files\Microsoft Security Essentials
Which schedules a quick scan, and checks for updates before the scan. change to ScanType 2 for Full scan.
looks like -SignatureUpdate is a viable command too