For some reason, my Visual Studio 2008 Express has started to throw compile errors when including <iostream>

All projects that previously compiled without errors are now failing.

In fact I can reproduce this by creating a project containing the single line #include <iostream>

The error I get is
c:\program files\microsoft visual studio 9.0\vc\include\xstring(31) : error C2143: syntax error : missing ',' before '<'

Reinstallation has had no effect.

I'm stuck! Any ideas?