Is there an NPC in the Xbox version of morrowind that allows you to regenerate other NPC's you've killed? If so, where abouts is he/she?

Nope, no such thing in the Xbox version.

is there on ein the comp version

not on the xbox version but on the PC version you can use a console command

there are 2 ways to do this

1)

you just killed someone, say a hlaalu guard, press the ~ key and click on the guard's body...now in the console command type: resurrect

now the NPC is alive

2) this one requires you to know the Unique ID of the NPC that the TES construction set gives (this is included with the game)

so for a hlaalu guard it would be something like guard_hlaalu or hlaalu_guard...lets say it was hlaalu_guard

now press the ~ key and type placeatpc "hlaalu_guard" 1,1,1

the guard will appear infront of your character

this can be done on anyone, including Vivec himself