Cavalry and Xbows are both things that one can hybrid easily, but HXbow is not. The points necessary in Horse Archery require so much Agility you simply can't have the Strength to pull of a decent Infantry build to accompany it.
Your choices are either a 12/24 build that can pull off shooting while on a horse semi-decent but is extremely limited as infantry or a 18/18 build that is more useful as a melee build on foot and on horseback but mediocre as a shooter while mounted.
Some examples:
Level 30 (4 420 577 xp)
Strength: 12
Agility: 24
Skills to attributes: 2
Power Strike: 4
Athletics: 8
Riding: 8
Horse Archery: 4
Weapon Master: 5
One Handed: 90
Crossbow: 130
Level 30 (4 420 577 xp)
Strength: 18
Agility: 18
Skills to attributes: 2
Ironflesh: 2
Power Strike: 6
Athletics: 6
Riding: 6
Horse Archery: 3
Weapon Master: 6
One Handed: 108
Crossbow: 130
I'm no expert of Horse Crossbows, but the best possible advice you could get regarding them would probably come from ROHYPNOL.