Ok. Here we go. This is the best real-time antivirus scanner around. AVast! comes in second, but MSE is lightweight and knows Windows better than any other antivirus sytem.
First things first. Uninstall your "old" scanner. I don't know which one you had so I'll just use Norton as an example.
Click on the start button (the windows logo located at the bottom left of your screen on the task bar).
In the search bar, type 'programs and features' then click on the programs and features icon.
in the programs and features windows, find Norton (or whatever antivirus you already have installed), single click on it, then click uninstall (it's located between organize and change)
Follow the uninstall process.
Reboot if necessary.
Now, click on the following link http://windows.microsoft.com/en-US/windows/products/security-essentials
select your version of windows 7 (most likely 64 bit)
Save the file.
Then run the file.
Take the defaults.
If all goes well, you'll soon be protected from malware by MSE. Yay you!