It might have to do something with "EditFlags" in the registry (search for it).
On my system, I experienced a massive speedup after I deleted all unwanted file extensions. Proof? Verified with RegMon - everytime a file is double clicked, ALL file extensions are examined thoroughly (throughout all their entries). Obviously, if you reduce their number, you gains speed.
File extensions are used for determining what to do when you double click on a file, but usually they can be opened from inside the program that understands the file type (but not always). Also, there are some filetypes 'reserved' from the system to block you from opening them (that's legitimate, because you can do nothing useful with them: .pnf, .sys...). Also, on clean install of XP some Office extensions are reserved (yeah, that's monopoly

) like for word, excel, and infopath or publisher (I don't remember which). Some are reserved just for the icon. There are also many entries to print them with right click (what's wrong with opening them in their editor and printing from there?).
Now I'm not telling you to go blindly delete all you see, there are problems when the wrong button is pushed.

Also, MANY programs re-register their types, so your efforts may be futile. And windows installer can pest you if it doesn't like what it sees (I've solved that, but will post it another time).
But I tell you, there is great speedup to be gained there.
GL