Assure that you are having sufficient free space in your OS drive....
If you right click on the scan schedule and click on scan now whether it is scanning?
Check this also
Click Start > Control Panel > gpedit.msc
Go to Windows settings> Security settings > Local policies> User rights assignment.
Edit Adjust Memory Quotas for a Process and Replace a Process Level Token, and add Network Service to the two policies.
Restart the computer after applying the changes.