Control ID's cannot be changed using properties window

[Brian Mitchell]

Whenever I add an object to a web page (such as a text box) I cannot change
the ID. For instance, if I change it from TextBox1 to txtUsername as soon as
I hit enter it changes back to TextBox1. This is happening on all my web
forms in this project. I can, however, change it in the HTML code and it
works fine; it seems to be isolated to the properties window of the
designer.

It's probably some option or I messed up the HTML code somehow, but if
anyone knows how to fix this I would be grateful.

Thanks!!!