For example, you might schedule scans to run at 8:00 P.M. If you select a four-hour time interval, scans on client computers start at a randomized time between 8:00 P.M. and 12:00 A.M.
To randomize scans to improve computer performance in virtualized environments
In the console, open a Virus and Spyware Protection policy.
Under Windows Settings, click Administrator-defined Scans.
Create a new scheduled scan or select an existing scheduled scan to edit.
In the Add Scheduled Scan or Edit Scheduled Scan dialog box, click the Schedule tab.
Under Scanning Schedule, select how often the scan should run.
Under Scan Duration, check Scan for up to and select the number of hours. The number of hours controls the time interval during which scans are randomized.
Make sure that you enable Randomize scan start time within this period (recommended in VMs).
Make sure that you apply the policy to the group that includes the computers that run Virtual Machines.