This is the new name for Microsoft Security Essentials (MSE).
— old name, new use.
Maybe not ideal in a ‘corporate’ situation (like schools) but maybe a good one to have set up at home.
The new version (still in beta version) can be used on a bootable drive (eg CD or USB) so doesn’t need Windows to be working (or even installed) – so it could help you recover a ‘broken’ computer. Early reports seem to suggest that WDO can find problems that MSE misses especially useful with the dreaded and secretive ‘rootkits’.
The download is a 200Mb+ file but may be worth getting hold of and adding to your security kit.