I know that there is no TI99/4A emulator for XBOX but it would be a great idea for an XBOX homebrew programmer to bring a TI99/4A emu to XBOX.
Currently, there is a freeware TI99/4A (also emulates the TI99) Emulator called Classic99. It is located at
http://www.harmlessl...earch=Classic99 & the source code is included in the download. This emu is being updated many times so this emu changes all the time. The emu works good but not all features have been implemented. For example, speech is unimplemented which means that you can play games such as Parsec but you won't be able to get the speech until it is implemented.
The Mess emu located at
http://www.mess.org/ is also a better idea because it supports many systems such as TI99, TI99/4A, Fairchild Channel F, Watara Supervision, TRS-80, et al.
In fact, Mess emulates the TI99/4A & the speech is complete which means that you can play games such as Parsec & hear the speech.
A TI99/4A emu for XBOX would be great. I wish that some XBOX programmer would port MESS to XBOX since this emulates more systems.