I had an old POS emachine that had the motherboard go up in the summer. I am looking at picking up a cheapy desktop with no OS for less then $200, I have all of the external components I need, and before I tossed the old desktop, I took out the hard drive. When I get this new desktop, can't I just put the old hard drive in the new desktop?

Nope

Explain please

You know what? Screw it. I want a clean slate anyhow. I have the OS I can load.

*but tell me why I cant do it

You should be able to hook it up pretty easily, assuming they use the same interface (such as IDE cable/connection) and you set the jumper correctly.

I read that as "piece of shit, machine".

wait... thats not what it is?

It didn't work.

Because when you install an OS, it loads all the drivers for your specific hardware like m/board, processor and so on. If you just whack the drive into a new PC, it may boot up if you're lucky, but the system is going to be so unstable it's really not worth it. Rebuilding a PC only takes a couple of hours, just get on with it.

Yeah, that's what I did.