I just got an Acer 5315 from a friend of mine, he said he dropped it one day and when he turned it had a wierd message and wouldn't start. I played around with it and figured out it was the hard drive. I looked on the internet and saw that Acers have a reputation for having bad hard drives. I need to know if the voltage on a hard drive matters when getting a replacement. I have a Compaq cq50 laptop, which I was going to take the hard drive out of that and put it in the Acer to see if it worked, but the voltage on the compaq hard drive was different than the one that died in the Acer, and I didn't want to blow anything in my Compaqs hard drive. And also, is the basic size for laptop hard drives 2.5"? Thanks.
2.5" is the standard laptop size, yes. Make sure you get one with the correct interface as well - if it is an older computer, it's probably PATA, and if it's newer, it's probably SATA (I'm not familiar enough with Acer notebooks to tell you which). If you remove the old hard drive and it has a connector with a ton of tiny pins, it's PATA. Otherwise, it's SATA.
As for voltage, that's part of the interface standard. As long as you get the right drive interface (PATA vs SATA), you should be fine.