HTTP Compression Problem

I have enabled HTTP Compression on my IIS 5.0 Server and selected "compress
static files" only. I have also configured the metabase to compress the "dll"
ext for static files.

i.e. cscript.exe adsutil.vbs set
W3Svc/Filters/Compression/GZIP/HcFileExtensions "txt" "dat" "xml" "dll"

Problem is IIS is compressing an ISAPI DLL that I already had installed.
(this is an ISAPI_Redirector dll for apache) I can see the dll file in "IIS
Temporary Compressed Files" folder.

This problem does not occur on IIS 6.0 (Win2003 Server) only on my IIS 5.0
(Win 2000 Server)

Any ideas?