The menus generally are controlled in the Registry, but I don't know (and don't find at Microsoft) anything about where or how to correct this problem - if it is a reg problem. There are a couple of corrections for some vaguely similar things, but with warnings that the "keys are all different" so that you can't extrapolate to this.

It is possible for an Administrator, in WinXP, to edit the menus that different groups of users can see, and it's possible that someone may have applied a "group policy" that removed this menu item for you. That would require, of course, that someone with Administrator privies has had access to your computer or to a server it runs from, and also that the "Administrator" knew how to do it (or didn't know what (s)he was doing).

IF THE COMPUTER has had "Group Policies" set up on it, a change in either your group priviledges or in the "Sharing" for the folders/files you're looking at could change the menu you see. If, as is normal for a single-user at home, there are no policies specifically set up, even a change in sharing shouldn't affect what menu you see - there would just be some things on the menu that wouldn't work for you.

Security patches from Microsoft are always a possible culprit when things change, and recent patches in particular sometimes have resulted in changes that affect "user priviledges" in unexpected ways. I don't know of any changes that should do what you've indicated, but if you right click and select properties, then look in the "Security" tab, you can see if your use has been changed, and/or if new categories of users with different "authority" have perhaps been added that have pushed your "user class" down on the list. While the menu shouldn't(?) change for you, what you're allowed to do may have been tweaked.

If you've had the System Restore function turned on, at the same "Properties" location you may find a "Previous Versions" tab where you could click back to an earlier setup that shows the menu as expected. If you consider doing this, it would be good to try to remember when things changed so that you can choose a backup just prior to when it happened to restore to the most recent good one.

As always of course, when unexpected changes pop up, you have run a full AV scan and all your good AntiMalware checks?

John