Thanks a lot people. I think the main problem is time, because if I try and return the HDD, I've effectively lost my main PC. A reinstall of XP would take ages - I'm not sure where all of my CDs/DVDs are. Because of the problem, I've backed up the HDD so if it suddenly goes, that's not a big problem. I ran the Lifeguard diagnostic utility, and it reported no errors, including the SMART test, but I haven't run the full 2+ hour test yet. I did start it for 1/2 an hour, but I needed to use the PC, so I cancelled it - I must find a 2+ hour slot when I don't need the PC!
I can't remember what utility I used, it may have been from the Ultimate boot CD, but several months ago I ran a very long HDD diagnostic utility on the drive which took 2+ hours, and it reported no errors.
Eventually, what I'll do is wipe the HDD and reinstall Windows XP. If the problem returns, I'll upgrade this 7200rpm SATA disk drive, and get a solid state drive, and put Windows 7 on it. That's my plan of action. Thanks people.