how to get back microsoft security essentials?

First of all, did you try restoring your computer to a previous time? System Restore on Windows often helps in restoring lost files on a crashed computer. I think you should try doing a system restore before downloading all the software you had previously and installing them again on your PC.

To do a system restore on your PC, if you are having a Windows XP based one, then please follow the following steps,

Step 1: Press 'Start', go to 'All programs', then 'Accessories', 'System Tools' and 'System Restore'.

Step 2: When 'System Restore' opens, it will show a window called 'Welcome to System Restore'. Select the first option on the right side of this window, called 'Restore my computer to an earlier time' and click 'Next'.

Step 3: On the next screen, you will prompted to select a date from the past when your computer was working perfectly. Choose a date from the calender and press 'Next'. Windows will automatically restore your computer to the previous state and restart it. Upon restart, you should be having your installed programs back.

On the other hand, if you are using Windows 7 or Vista, follow these steps,

Step 1: Press 'Start' and in the search box type 'System Restore' without quotes. From the results, click on 'System Restore'.

Step 2: Upon opening of 'System Restore' window, select 'Next'. And then choose the date you want to go back to from the list of dates given there. Press 'Next' again after choosing.

Step 3: Windows will automatically restore your system to a previous date as you've chosen and your should find your previous programs there.

If 'System Restore' does not work and you want to download Microsoft Security Essentials again, then visit official Microsoft website,  select Microsoft security essentials and click on 'Download it free today'. It will ask for your language and operating system type, choose accordingly and the download will start.