I have an Ebony bow equipped on my character.. for some reason, when i try to unequip it, it will first say: "Ebony bow unequipped". Right after that, it will say: "you cannot unequip that item" or something along those lines..
when i try equipping a weapon, it won't equip period. It is realllllly starting to piss me the [censored] off. someone help me before i throw my [censored] laptop out the [censored] back yard, please. >=[
please excuse my language. too [censored] frustrated at the moment.

Running any mods? I don't recall any ebony bows being quest items.....

The game won't let you swap weapons while attacking, and it isn't always the smartest about when, or if, you are attacking or not...... if the problem presents itself again, save your game, exit, fire it back up, and reload your save. See if that helps.

If using the PC version, theres a good console command tutorial http://www.uesp.net/wiki/Oblivion:Console_Command_Tutorial which explains better than most how to get stuff done, and one of the solutions might just be typing

player.removeitem 35e7b 1;
or
Player.RemoveItem 35E7B 1;
or
Player.RemoveItem 00035E7B 1;

Thats only the code for a non-enchanted ebony bow, if its enchanted, you can look the code up from the link above just as easily, but, if it was enchanted by you at the alters, its probably too much effort to track that code down, so youd be better hunting thru other commands to try

No mods whatsoever, only DLC... i also am running vista.. so no console -_-
wierd thing happened though.. i used the summoning stones to get daedra weapons, thinking it might help.. well, when the weapons i summoned had all had their time run out, i could equip any weapon i wanted again, but the ebony stays equipped regardless of what i have equipped -_-
Also, if i do that and reload, it reverts back to being stuck with the stupid bow.. i don't wanna restart the game, but i think i might have to reinstall god damnit.