Hi PurtyQwerty
With a failing hard disk, your first priority is to back up your most important data - which of course you should have been doing anyway! But if not, DO IT NOW - BEFORE you start trying to install the new "C Drive" disk. Copy the relevant folders (eg My Documents, Mt Image, My Videos etc, plus your email files and any other important user files) onto alterative media like one or more data sticks. If you have a huge amount of data, it may be cheaper to buy 2 HDDs and, for a laptop, a remote enclosure – then you will have a back up facility for the future. (see below)
Choose the replacement HDD you need. Depending on your machine this could be 3.5inch (desktop) or 2.5inch (laptop). If the machine is more than about 12 years old, the HDD may have a PATA/IDE (parallel) interface. More recent models will have a SATA (serial) interface. So check out the physical size and interface type – then decide on the capacity. You can take the opportunity to install a higher capacity drive with a faster spin speed than the existing one if you like. Assuming your interface is SATA, you may also consider purchasing an SSD (solid state drive) as the replacement HDD – more expensive but faster and no delicate mechanicals to worry about.
I’m going to assume you have a laptop PC that’s about four years old – in which case you’ll almost certainly need a 2.5inch SATA drive. Make sure you don’t buy an mSATA device – these are much smaller and the plug won’t fit your machine! You’ll also need to buy or borrow an external disk enclosure with USB lead - suitable for the type of disk you have. This is essential for the cloning processs on a laptop as well as for backing up files later.
If you have a desktop with a spare drive bay and power/SATA interface lead, then you won’t of course need an enclosure and will just need a new 3.5 inch disk (or two – one replacement for the C: drive and one for future back-ups)..
Providing your machine is working normally (apart from that message on boot-up), you can make an exact copy of the HDD onto your new one, Most disk manufacturers provide a free cut down version of Acronis True Image (or Norton Ghost or similar) to download which allows you to copy everything on your existing disk onto your new disk. (It's called "cloning" the disk.) . I believe recent versions of Windows (viz: win7 onwards) also have this facility built-in to their Backup Tools area. You could alternatively download Macrium Reflect Free ) which will let you create a clone.
The result of Cloning is a disk that is identical to your existing hard drive - including the Master Boot Record and with more empty space if you decide to buy a larger disk than you had before. You’ll find step by step instructions for cloning at http://pagestart.com which provides versions for windows XP and later.. For earlier versions of windows, the instructions for XP will suffice. For Win7 onwards you need to make sure that both partitions are included – this is explained on the pagestart site.
When cloning is finished, you’ll need to physically remove the old disk and install the new one. Turn off power to the machine by pulling out the power lead. On a laptop, you must also remove the battery then press and hold the start button for at least 10 seconds to help discharge any capacitors inside. Now follow the manufacturer’s instructions (HP Maintenance and Service Guide) to remove the old disk. Although many technicians don’t bother, it’s wise to take electrostatic precautions while handling disks – basically bond yourself electrically to the metal frame of the machine with an electrostatic wrist strap and avoid touching the circuit board and metal connectors of the disk and inside the laptop– hold the disk by its long edges and connectors byt their plastic parts.
It's not a difficult job - work methodically, be careful and take your time. YOu should end up with a machine that works better and faster than before.
Good luck!