1. Open "Control Panel" ->"Users and Passwords"
Select the "Advanced" Tab
Press the "Advanced Button"
Select Users from the left Frame
Right-Click on Guests in the Right Frame
Select Properties
Uncheck "Account is Disabled"
Press "OK" and reboot.

2. If you're using Microsoft Outlook, make sure to install the latest Service pack for your version.

3. Scan for malware using both of the free programs below:

Malwarebytes Antimalware

SUPERAntispyware Removal Tool

Hope this helps.