A typical caster can't use mage hand to make attacks
The 0-level Sor/Wiz spell mage hand [trans] (Player's Handbook 249 and also for Pathfinder here) has the entry Duration: Concentration. So, after the caster takes a standard action to cast the spell, each turn the caster must take a standard action to maintain the spell, and that leaves the typical caster without the standard action necessary to make attacks with whatever the mage hand spell effect is toting: the caster essentially spends his standard action solely to move around the affected object (just as the spell indicates) rather than spending his standard action to use the affected object violently like he could naturally with an effect like, for example, the spell telekinesis.
Thus, in the same way a typical creature can't take a move action to make an attack, a caster can't just move an item that's affected by the mage hand spell so that the affected object makes an attack. Like this fine answer suggests, dropping an affected object on a foe and using an object to trigger a trap are likely the best ways for a typical caster to use the mage hand spell in combat.
"But what if he can?"
This DM would rule that a caster that employs the spell mage hand to affect a less-than-5-lb. mundane weapon, keeps that weapon within the mage hand spell's close range, and somehow acquires on his turn a second standard action (or somehow manages to concentrate on a spell using an action other than the caster's standard action) could, in fact, make with that weapon one attack… or, if the caster's actions allowed, even make multiple attacks with that weapon.
In such a case, this DM would have the caster make the attack roll using the caster's base attack bonus and, instead of the caster's Strength or Dexterity modifier, the mage hand spell's caster's spellcasting key ability (much like the using the violent thrust version of the aforementioned telekinesis spell). However, the mage hand spell can only carry 5 lbs., and, according to carrying capacity, that puts its Strength score between 1 and 2. Because of this, this DM would rule a weapon the mage hand spell effect employs suffers a −4 penalty on damage rolls.
Further, as the weapon isn't being wielded by a creature, the weapon doesn't threaten an area, therefore it can't, for example, help an ally flank a foe or make attacks of opportunity. (This DM would likely look to the spell spiritual weapon for guidance, that spell also seeing a weapon attacking on its own accord at a distance yet at the caster's behest.)
Seriously, though, in this case, this reader believes that a caster who has available on his turn two or more standard actions should be able to find something better to do with those standard actions than to use the first to concentrate on the mage hand spell and the second to make an attack with whatever that mage hand effect's carrying! I mean, c'mon: You're a wizard with two standard actions per turn: Stop toying with your foes and end this already!
