Vista - .NET Fx 3.0 installation fails with a 16-bit MS-DOS subsystem erro

Hi everyone,

My application's NSIS based installer runs dotnetfx3.exe before it runs
msiexec to install the actual application.

On a large number of XP computers, dotnetfx3.exe fails about half way
through with a 16-bit MS-DOS subsystem error.

The message box that appears claims:

C:\DOCUME~1\USER\LOCALS~1\Temp\DOTNET~1.EXE
NTVDM CPU contains an invalid instruction
CS:0fa1 ... Click Close to terminate the application.

I have been unable to tie the problem to any particular XP build,
architecture, CPU, locale, ...

Can anyone think of what this could be?

Thanks in advance!