Computer Reboot During System Restore

My grandfather has given me his computer to fix. The only problem is, he doesn't want to lose everything, though I think he's going to have to (or already has). He was doing a system restore (unnecessarily, may I add) and switched off the computer because it was taking too long (!). Now, Windows XP Home will not boot. The XP loading screen is displayed, then a blue screen for a split second, then it reboots. Safe mode does the same, and last known good configuration does not go past about 1/6 of the white loading bar at the bottom of the screen.So I thought I'd reinstall XP over the top. The problem is that the partition is now 'unknown', according to the Windows XP Setup, so it would appear the partition data is messed up. Thus, I cannot install XP without formatting.