Internal Hard Disk Drives (HDD's) fail all the time.
Just to give you a quick lesson, inside HDD is a platter (some have several depending on capacity and design). The platter is essentially a spinning disk, most drives spin between 5400 rpm (rotations per minute) to 7200 rpm. Some performance drives can go up 15000 rpm. Your data is stored on the platter(s), and a mechanical arm which also moves very quickly to read or write data on those platters. The inherent problem with HDD's is that because it rely's on mechanical parts it can be subject to failure from shock, vibration, or just wear. Unfortunately you can't predict a drive failure, sure there are built in warnings (like SMART tests or POST tests) and watching for big performance issues. You can't really predict when a drive is or going to fail.
Also HDD's are considered consumable parts, meaning more or less that they are expected to fail at some point.
If you wish check out
http://en.wikipedia.org/wiki/Hard_disk_drive and the article will give a lot more infromation about how HDD's came about and function.
With that said, repairing a hard drive in a computer is like putting new brakes on a car. You are going to have to do it eventually, the one other big difference that only you can determine the value of your data and so create a backup plan accordingly.
In terms of buying another computer, from my experience. If you spend 800+ usually Lenovo or HP Business PC's (NOT CONSUMER) are the best and/or have better customer service situations.
If you are spending less than 800, generally any pc will do. Most PC's sold for less than 800 have one thing in mind. How little can we put into these to make as much money as possible. I have seen way too many $299 notebooks that are built from refurbished parts as new machines. I find that the $299 "door-buster" computers have problems or are built from remanufactured/refurbished parts. Basically these are throw away computers, I hate to say it but it's just the way the business has headed the last 5 years.
With all that said, I would say that you should ask someone that has experience with computers. Working with them directly, and get a feel on your own of what experiences people have had. If you go to any retail store and speak to anyone is Sales, take anything said with a grain of salt. I know that sounds cynical, but trust me most retail stores don't train there associates well and anything they are trained on is in a way that tries to make them more money.
I know this must sound more like a rant than advice, but thats my 2 cents for you.
Bottom line advice, fix the mac it will be worth more 3 years from now than a PC of similar ago.
