I sucessfully installed Microsoft Security Essentials on my Compaq D51S Desktop, P4 and 2 GB of memory which has a fresh install of XP Pro fully patched and all hot fixes applied. The desktop did not have Windows Defender installed or any other AV or Firewall. After the install of Essentials with the latest definitions installed, I did a full disk scan with no malware or viruses detected. I then decided to 'disable real time detection' but opted for a full scan every twenty four hours. After several hours, I noticed that the desktop was at %100 CPU utilization. I checked 'Task Manager' and the only abnormal process was MS Essentials. I also looked closely for any process I did not recognize, and also at 'svchost' processes which were normal. I also use a opensource called svchost viewer tool from http://svchostviewer.codeplex.com, which gives provides a detailed list of what an 'svchost' process is connected to. The desktop became more and more unresponsive, but I was able to reboot from the "Start Menu'. After rebooting, the desktop became unresponsive to the point of necessitating a hard boot. I reboot several times, regained control to the point that I was able to use 'Add Remove Programs' from the 'Control Panel', and removed Essentials. After removing Essentials, and the high CPU usage problem non-existent, I attempted to open a 'Word Document' and received the following error pop-up box . I attempted to create a new Word document, and received the same error. The weirdest thing was that I could create or open an attached Word document in Outlook 2007, which uses Word 2007 as the message editor. I then did a 'repair install' of Office 2007, navigated to Office Updates, and Windows Updates to make sure that no hot fixes or patches had been removed with the 'repair install' process. The only patch that was missing was Office 2007 SP2 which I reinstalled and rebooted. After reboot, I again attempted to open or create a Word document, and received the same error message. I decided to do a 'remove' Office 2007 using the DVD. This is when everything really went to 'Hell'. After removing Office 2007 and a reboot, I attempted to do a 'Install' of Office 2007. After accepting the product key, after several minutes the Office installer stopped with an error. I tried a couple of more times to install Office 2007, but continuously get an installer error. I know what followed at this point is a full OS install, but I decided to attempt an install of Office 2003 which fails with an 'incorrect product key'. My XP, Office 2007, and Office 2003 are valid purchased MS products. I' m making the assumption that once the Office 2007 service packs and patches are installed, their is no longer a downgrade path from Office 2007 to Office 2003. I' am a Software Quality Engineer by profession. I can fully sympathize with Microsoft Software Quality Assurance and System Test Engineers. They have an enormous software product base to support, as well as a huge hardware platform base to support and perform regression test with any new product base release or software hot fix/patches/service pack release. Microsoft has acknowledged that it took a 'big black eye' with Vista and its release to the public for consumption. I hope for their sake, that Essentials does not 'blacken the other eye'.