MSE is available for Windows XP 32 bit ONLY and does not support Windows XP
64 bit, although it makes no mention of this on it's system requirements page.:silly:
http://social.answers.microsoft.com/Forums/en-US/msestart/thread/d06e999b-56f8-418b-911d-2ff1ad78eea5/ Edit to add: There is the possibility you may be (unwittingly) using a "pirated" copy of XP:
http://news.techworld.com/security/3203037/microsoft-blocks-pirates-from-security-essentials-antivirus/Microsoft blocks pirates from Security Essentials antivirus
Free software installs only on legit Windows
By Gregg Keizer | Computerworld US
Published: 10:38 GMT, 01 October 09
Microsoft will block users running counterfeit copies of Windows from installing the free Security Essentials antivirus software, the head of the company's anti-piracy group has said.
Security Essentials, which launched early Tuesday, is basic anti-virus and anti-spyware software that Microsoft touts as suitable for users who can't, or won't, pay for security.
"During installation, you'll be asked to validate Windows running on your PC to make sure that it's genuine," said Alex Kochis, director of Microsoft's Genuine Windows team, in a post to a company blog Tuesday. Genuine Windows is the umbrella label for several of the company's anti-piracy technologies, including product activation and the often-criticised validation and notification components, which regularly determine whether the copy of Windows running on a PC is legitimate