Best rootkit scanner external drive

My mom's computer (Win XP) got what appeared to be a rootkit infection (deeply embedded browser hijacks, inability to do a system restore or windows update,...). The system was slow and bloated before the infection, so I opted to do a cold OS re-install (Media Center edition) after wiping the boot disk (DBAN), flashing the latest bios, and resetting the router. She has an external drive which she used for backing up My Documents, and would like to get some data off of it, but she's on strict orders not to re-connect it to the system until I figure out the best way to scan it and make sure nothing on it can re-infect the computer.

After the OS install I put AVG Free on it. Seeing as how she had that on her system before and still managed to get infected, I'm not counting on that to be the silver bullet, and I don't trust it to scan the external drive. (It didn't help that she was downloading stuff she found by googling 'free games'. I think she's learned her lesson now. Parents.)

I've seen references that certain apps, like TDSSkiller, may only scan the boot disk. Not sure if that's accurate. So what would you recommend I use to verify the external drive is clean?

 

Thanks, TimW. Just got around to scanning her external disk this past weekend. Neither scanner found anything malevolent, so I told her she could go ahead and copy stuff back to her system.