Don't browse the web using an Administrator account. Use a Standard user account to prevent malware from making system-level changes.
Some reputable providers still offer legacy versions of their software for Windows 7 that are more frequently updated than MSE. security essentials windows 7 download
On older hardware, MSE can sometimes cause significant slowdowns during "Real-time protection" scans. Better Alternatives for Windows 7 Users Don't browse the web using an Administrator account