As others have said, probably the best way to proceed is to avoid going the wargame-route, and instead using the battle as a backdrop for the heroic actions of the PCs. I use a system adapted from the Dungeon adventure "Foundations of Flame" (I don't know what issue that was in off hand, but it's also in the Shackled City hardcover). It's very similar to the system in "Heroes of Battle", but I think it's a bit simpler. Certainly, it doesn't take as long to describe
The first thing to do is to consider the forces available, and the victory conditions for each. So, taking the Battle of Helm's Deep as our example, and considering Aragorn, Legolas and Gimli as the PCs (and using the film version of the battle where there's a conflict), we have 300 men, a few hundred elven archers, and the PCs inside Helm's Deep (powerful terrain advantage), versus 10,000 uruk-hai and their assorted war machines. The PCs win if they hold the fortress until daybreak (let's say 10 hours), the uruks if they kill all the men, or they break down the fortress walls within those 10 hours.
The next thing to do is to prepare a timeline of events, assuming the PCs do nothing heroic. So, if the PCs take their position in the battle, but are otherwise unremarkable, what happens? In the example, I would rule that the uruks break down the walls in 8 hours. (It's usually best if the outcome favours the opponents of the PCs. That way, the heroic actions of the PCs matter most.)
I find it's helpful to set up a numeric count of victory points across time for each side. Once a victory point total is reached, the battle is over, and one side as won. In this case, the uruks need 100 VP to win, and gain 13 per hour. The PCs win purely by preventing this - they don't accumulate VP in this case.
Next, work up a list of things that the PCs can do to change the outcome. These are things that the PCs have to do - merely stating that they will shore up the fortifications is meaningless, since any competent commander will do that anyway. However, if the dwarf PC can bring his particular engineering skills to the task, that's another matter. Likewise, if the sorcerer calls in that favour he's been holding for months, and has a dragon join his forces, that's worth quite a lot. Other possibilities include single combat against the enemy champion, a rousing speech from the party bard, and so on.
For each of these events, work out a method of resolution (stats for the enemy, a Perform check DC, whatever), and the effect on the number of VP the PCs gain, or the number of VP their enemies get. Make sure you generate enough events for the PCs to swing the battle suitably (how much impact the PCs can have is up to you. I generally favour allowing them to swing a battle from crushing defeat to narrow victory, or from narrow defeat to sweeping victory).
Finally, create some "chaos of battle" moments - either bits of narrative where you will describe what happens to the PCs allies, scenes where the PCs battle run-of-the-mill enemy soldiers, or whatever. Note that, if there are any named NPCs you want to kill off, you really should create a scene where this NPC is shown dying. This may either be a death the PCs can prevent (in which case it's a heroic action), or one they cannot, in which case it fits here. Either way, you should show the death of any NPC who has a name. It's just the done thing.
Then, run it. Have a period leading to the battle, then run the "chaos of battle" moments and the "heroic action" moments in an arbitrary order. Make sure the PCs have plenty of opportunity for heroism, and don't get bogged down in the details. And, if things start getting too much for you, have a bunch of orcs charge the PCs position, and run the combat on autopilot while you think of what to do next.
Oh, one more thing: prepare a map of the battlefield, and some small markers for the positions of major units. Use this to keep track of where the PCs are on the field of battle. This should help keep things organised during play.
Edit: Couple of things I forgot: For a battle the size of Helm's Deep, I would probably want to create a dozen or so "Heroic Actions", and require the PCs to succeed at 8 or so of them. Also, you need to allow for the possibility that the PCs might make their own "Heroic Actions", and adapt accordingly. For each non-combat action, I would give XP as though for a monster of CR equal to the average party level. Combat actions give out XP normally, of course.