I am getting a new PC and I want to transfer my hdd from my exisiting pc (with the os on it) to the new PC. Both have sata ports. Is it just the case of plugging it in and booting or do I have to do other things?
It's actually difficult to do this. The problem is that, when the OS was installed on that HDD in your old machine, it was customized for it. That is, the Install process surveyed the machine and detected all the hardware devices it contained, then made sure to install the device drivers for them. "Hardware devices" includes a whole bunch of components of your mobo like USB ports, eSATA ports, SATA ports, video chip (maybe), audio devices, etc., PLUS any PCI cards you have in the machine. If you put that HDD into a new machine, there are two kinds of mismatch. First, it has drivers for many devices not present in the new machine. More importantly, it does NOT have the device drivers for many of the devices that are part of the new machine, and so it can't use them. This often results in failure to boot.
If you have your original OS Install disk, there is a process that MIGHT work. When you first turn on the system, you place your old Install disk in the optical drive and boot from there. You watch carefully and do NOT do a normal Install. Instead you choose a Repair Install. This is supposed to detect and fix all those driver mismatches and get you running smoothly. Sometimes this works, sometimes not.
The alternative, of course, is to back up everything on some other device, then put your old HDD in the new machine, wipe it clean and do a fresh Install from the original CD. However, then you have to re-install all your applications software, and finally restore all your data files from the backup device. A long process, but sure to give you a smooth-running system.
It's generally better to do a fresh install of Windows. You won't have as many problems and it's the easiest, more efficient way for a new machine. And plus your new machine will run at it's best performance.
Your better off getting a new OS all together, as with any computer a fresh install of windows is your ideal course of action. You can either buy a full version of Win 7 and upgrade to Win 8 if it appeals to you or you can get Ubuntu/Linux. The choice is yours, unless someone else has a better option those are the only options you pretty much have. You'll need a Win 7 disk anyway. Windows 7 Premium $179.99
It's up to you whether its worth getting or to get Linux. I would avoid getting any used or seller copies of an OS since you can end up with a counterfeit (non Genuine) version. If you do choose Win 7 check Windows 7 Versions Explained to see which version appeals to you. If you can't decide yet or unable to buy Win 7 yet, your best off with getting Ubuntu to hold you over. But really why get a new PC if you can't afford the OS?
You have 30 days of validation. Not sure what you mean by through USB though unless your talking about Linux. Linux can run from cd, hard drive or flash drive. I have heard of users using usb, floppy or anything other then hard drive when using Windows but this is ancient history when i've seen people do this with Win 95/98.
It's free to use and you can try it on your current PC (assuming you have the right specs for it) from USB or Cd to get familiar with it and that will give you time to get the money together for Windows.
Edit: you could also get Win 8 from the MS site which is like a demo version.