After April 8, Windows XP computers will be more susceptible to malware and viruses beginning, since Microsoft will no longer address major holes in the software. Although antivirus software will continue to fend off some malicious attacks, Microsoft's security updates provide an essential line of defense.
Running Windows XP SP3 and Office 2003 in your environment after their end of support date may expose your company to potential risks, such as:
Security & Compliance Risks: Unsupported and unpatched environments are vulnerable to security risks. This may result in an officially recognized control failure by an internal or external audit body, leading to suspension of certifications, and/or public notification of the organization’s inability to maintain its systems and customer information.
Lack of Independent Software Vendor (ISV) & Hardware Manufacturers support: A recent industry report from Gartner Research suggests "many independent software vendors (ISVs) are unlikely to support new versions of applications on Windows XP in 2011; in 2012, it will become common." And it may stifle access to hardware innovation: Gartner Research further notes that in 2012, most PC hardware manufacturers will stop supporting Windows XP on the majority of their new PC models.
Though MSE will have another year of support.
The post details the products in question as well: "For enterprise customers, this applies to System Center Endpoint Protection, Forefront Client Security, Forefront Endpoint Protection and Windows Intune running on Windows XP. For consumers, this applies to Microsoft Security Essentials."