Before we discuss counter builds, let's talk about what an early Zealot rush is:
"my defenders got wiped by 4-5 zealots and more and more zealots kept pouring in and eventually killed me"
This is the dreaded 2 Gate rush.
Your first goal is going to be identifying it. Luckily this is fairly easy to do. If you scout with your 12-13th drone/probe/scv you should get to your opponents base while his first Gateway is still building, worst case while his second is. All 2 Gate rushes start the same: Pylon, Gateway, Gateway, Pylon. This makes them very easy to pin down. If you see that second Gateway go up before the Cybernetics Core you know he's playing some sort of rush (pro tip: you can click on a Protoss building warping in to see what it will be). If he does a delayed rush, chances are you'll already be in a better position to defend, so you really want to watch out for this early rush.
The attack that will come should be 5 Zealots with 2 more in route (obviously he may wait for more or less, but this is the best time for him to attack).
As Zerg a 2 Gate opening is one of the scariest things to deal with (right after Hellion harass into Mech). If you fast expanded, use the expo as a damage soak and cancel it before it gets killed (if you can). Ideally you'll want to keep that expansion alive as his economy will already be devastated. When you scout a 2 Gate, you'll want to build a Spine Crawler almost immediately. Additionally, a Roach Warren is also recommended. Roaches are very strong vs Zealots (slow attack and starting armor vs a unit with fast attack) and if you can position some by your ramp or other choke you can cut the surface area of your Roaches in half. If you're already into a more speedling based build, then you're going to want to wait for him to surround your Hatchery or engage your Spine Crawler. This will naturally give you a very nice surround on his units as they will spread out. I also recommend bringing your Queen along for ranged dps (and to infuse the Spine Crawler).
The key thing for Zerg about repelling a 2 Gate play is the recovery. Early 2 Gate is so effective that even though he is putting himself at an economic disadvantage, he may do enough damage to your economy to win. You'll want to do the following: Put an Zergling outside his based (this is a scout for his next move); Tech to Roach or Hydra or Muta, as 2 Gate will likely lead into 3 Gate, 3 Gate + Robo or 4 Gate; Power drones, this one is key as he's making a huge economic sacrifice to push with 2 Gate and you need to exploit this for victory.
Terran traditionally have less problems with 2 Gate as they can usually wall off before the Zealots get there. If you already have Vespean Gas then its ok to go for Marauders with Concussive Shell, however, if your strategy didn't include that, you're going to want to skip add-ons and throw down a second Barracks when you scout the second Gateway. With Marines faster build time, you can create a tough Marine force behind your wall well before his Zealots arrive. This ruins the fact that he made an early sacrifice to go for a Zealot push and should give you command of the match. From here is very easy to transition into an MMM or even BioMech which should give you a quick victory.
As Protoss you're going to also want to wall, stick a Zealot or two in the opening and push for Stalkers. If you can have enough 1 Stalker out and your opening plugged before he gets there, chances are you've already won. It is not unusual, upon seeing a 2 Gate, to throw down a second gateway yourself. As defender you'll naturally have the advantage as his troops have a longer rally distance, and you can control the choke. If you push out an early Sentry, he'll probably Rage Quit and call you a cheater.
Now if you're dealing with a Proxy Gate (which is different than a 2 Gate) you have slightly different things to watch for. Lack of Assimilators and Pylons in his base are your best clue. At this point you'll want to start taking your scout and covering all the corners of the map. Additionally move a scout to outside your natural or the nearest Xel Naga tower.
Are the Protoss fairly micro heavy in the early game?
I agree with other posters that Protoss benefit from micro, but that so do all the races. Protoss do get an early "caster" unit in the Sentry, and do benefit from micro to save wounded units since their shields recharge, but then zerglings benefit from careful surrounds and marines benefit from grouping and stutter-stepping.
It's true that you see pro players using forcefields to incredible effect, but you'll also see them do amazing things with reaper scouting/harass, expand-blocking with workers, and so on. Use these tactics if you can, but they're by no means required unless you're in the top levels. At my level I rarely see sentries, let alone good forcefield usage.
Can an underpowered computer limit your ability to micro in a game?
Sure, but if your PC is lagging enough to mess up your micro in the first battle of 10 vs 10 units, it will be completely unplayable in the later stages of the game. Therefore if the game is still playable in the late phases when you have big armies, you should have no problems early on. Later-game micro could be a problem, when you have tons of units, but that varies PC to PC and you just have to try it out.
Best Answer
Sentries, Sentries, Sentries!
I play as zerg and HATE sentries.
They're useful against early pushes to hold off a rush, or at least create a choke point. The idea is that you can slow down the attack long enough to get an extra warp-in of units. Also remember that although roaches are ranged, their range isn't all that high, so with any kind of choke point, half their roaches will just be running around in the back unable to deal any damage.
Into the mid-game it's even more effective in that you've got enough force field energy to cut armies in half, or at least shape the battlefield enough to let your immortals and colossus go to town.
Separately, you'll want to identify as early as possible that he's going roaches so that you can go a little heavier on stalkers as opposed to the zealots you'd want if he were heavier on zerglings