[citation][nom]f-gomes[/nom]Yes it is. It is based on Forefron Client Security, which is also IMO a solid AV. No AV is perfect, though, always gave 2 or 3 around your office in case your "main one" doesn't find a particular virus.[/citation]
Most security companies use the same scanning engine and definitions for their corporate & consumer antimalware products. Microsoft isn't unique in that respect, as McAfee, Symantec, et al work the same way. How Microsoft is unique is that they now use the same UI for all of their products. Microsoft Security Essentials, Forefront Endpoint Security (the predecessor, "Client Security", looks like Windows Defender), and Intune Endpoint Protection all use the same UI, engine, and defs. The only real difference is the add-on centralized management features for the corporate versions. Also, in the Diagnostics and Recovery Toolkit (DaRT), Standalone System Sweeper looks like the old Defender, but lacks on-access scanning, and includes antivirus.
Then there is the new Windows Defender included in Windows 8. It looks and works like MSE, but unlike previous versions of Defender, it now includes the antivirus out of the box, so Windows 8 will be the first Windows that isn't lacking any antimalware protection from the start. The reason they called it Windows Defender is because a) "Microsoft Security Essentials" is a long-winded name, and b) "Windows Defender" has been the name of the *included* protection software in previous Windows versions. MSE will be unnecessary to download if you have Windows 8. Third-party certified antimalware software will be listed in the new Windows Store though, but it's unknown whether it will be available as a downloadable Metro app (dunno if it will have enough low-level access as a sandboxed Metro app), or only advertised, since Desktop apps will not be installable from the Store (unless antitrust regulators step in and force them to).