SQL Server 2008 R2 Express - exe not a valid Win32 application

I'm downloading SQL Server 2008 R2 Express from the Microsoft website:

Under Database with Advanced Services, I select 32-bit.

Do you want to run or save this file? I choose [Run].

The publisher could not be verified. Are you sure you want to run this software? I choose [Run].

This error pops up immediately:

SQLEXPRADV_x86_ENU[1].exe is not a valid Win32 application

I tried downloading it last night for the first time but got the same error. SQL was never previously installed on this computer.

System specs:

WinXP Home Edition Version 2002 SP3 - 32-bit OS

2.53 GHz Processor

1.25 GB RAM

28.4 GB Hard-Disk space available

7200 RPM Hard-Disk Drive

DirectX Version: DirectX 9.0c

According to the Support page, WinXP (x86) SP 3, all editions except Starter Edition, are supported.

I also meet or exceed the hardware requirements:

What am I missing? Why won't it load?

Thanks for your help!