Got it working! Didn't actually try unplugging the drive. Not quite sure what cured the problem. FYI, burnt another copy of Hiren's Boot CD using Burn to CD rather than Windows Explorer, which loaded ok. Tried loading Mini XP Windows, which came up with blank desktop. Ran 'Quick check' from Western Digital disc test accessory, which came up with no errors. Tried Mini XP Windows again, which loaded correctly. Ran 'Windows Disc Check' which came up with no errors. Following these steps, successfully booted off hard disc using Windows 'normal mode'.
Thanks again for your help.