sounds like the window regulater is broken. it's a problem on some of the older Xterra's.
I have an '01 and I replaced 3 of them so far.
It's not hard.. really. take your time and do it when it's warm (so the door panels come off easier)
the reg. should be around $121.12 ,but ask of a deal because Nissan knows that there a problem.
I got mine for $88.41
Good Luck
Perry