You sound like you know what you're doing so dunno why you didn't back up his data on an external drive
he lent his external drive (that his Mum paid for) to a mate.....2 months ago......
then format his HDD and then reinstall XP? Sounds like there was more to it or else you wouldn't waste 5 hrs when you could have done a fresh install in half the time.
His hdd had one partition with OS, apps, and data....and we are talking a 250GB partition 96% full.
Initially I ran the virus trojan software at boot and in safe mode, and got rid of a lot....but several bugs were reinstalling themselves at boot or later...
So I decided to run antivirus and trojan software off another partition or hdd while the infected partition wasn't active...there wasn't another desktop in the house...if there was, I could have pulled his hdd and slaved it in the other and run antivirus.
So I split his one hdd into two partitions, and did a fresh install on the new partition...and booted into that....My thought being there was a chance his original partition was never going to be totally cleaned, so he would need a new install anyways, from which he could access his data. though he would need to reinstall all his cherished apps.
but he didn't have the computer's main cd with mobo and nic drivers....so no network....(which is why I hate doing computer work for non commercial users), so I had to mess around finding drivers from the crap ASUS website using another computer and mess with several before finding the right ones (that took about an hour).
once I had net connection, I downloaded latest antivirus etc. then ran it.
after running it, and cleaning up 20 odd viruses, I then tried to boot into the original partition (i'd made the hdd a multibooter)...I got in a couple of times before the login prompt wouldn't accept passwords and wouldn't let me in whatever I did. I could still get into the new partition though.
I've never struck an issue like that before, and tried all I could to log into windows....but no success. .
I hadn't run a restore or recovery operation on the old partition at that point, and decided a PC shop could do all that. though the shop obviously didn't understand the state of the partitions and didn't resolve the issue.
If his Mum wasn't a very good friend who I owed favours to, once I realized he didn't have drivers, I would have told him to take it to a PC shop.
BTW, this experience highlights why I don't put faith in antivirus software....rather, I rely on regular OS and data partition image backups. Nothing resolves issues like a restore from a clean image backup. I use Terabyte's Bootit and Image for Windows solely. They are extraordinary products. and do failsafe hot OS images.