Definitely hardware and I'd give it a 50+% chance of it being your hard disk (the C: drive). It sounds as if it's coming fast to the end of its useful. Top priority is to back up anything important you have on that disk: Your email folders, your personal documents and hard to replace photos, anything you'd miss. If you can get a back up of the whole drive, even better. BUT make sure the back up is on something external - USB sticks, external hard drive (you can pick one up for £50 and it will be useful in the future too - that would be my favourite option).
Then, secure in the knowledge that the wors you can suffer is hassle, but not loss, you can set about deciding about your replacement machine.