There is a very good blog writeup by Stephen Toulouse at Microsoft as to why WMF is not a “Critical” issue on the Windows 9x platform
It’s technical, but if you can wade through it, it’s well worth the read.
With WMF we want to be very clear: the Windows 9x platform is not vulnerable to any “Critical” attack vector. The reason Windows 9x is not vulnerable to a “Critical” attack vector is because an additional step exists in the Win9x platform: When not printing to a printer, applications will simply never process the SetAbortProc record. Although the vulnerable code does exist in the Win9x platform, all “Critical” attack vectors are blocked by this additional step. The remaining attack vectors that we have identified require extensive user interaction and are not rated “Critical”. Again the “Critical” rating refers to code execution attacks that could result in automated attacks requiring little or no user interaction.