Log on/Log off Loop

[Voltron-IU]

I have a user running XP with SP3 and the other day, they attached a USB flash drive and may have removed it, before the OS finished installing it. They did NOT use the “safely remove hardware” option, from the system tray. Now, the computer boots up; however, no one can log onto the computer, for as soon as a user logs on, the computer promptly logs the user off, regardless of who the user is. I know this mimics the Virut virus/worm; however, the user claims the USB drive scans clean and he/she never saw a virus/worm notice pop up. The PC is secured with the latest version of Microsoft Security Essentials and the OS was fully patched. I would appreciate it if someone could point me in a direction, to allow us to log back onto this PC. Safe mode does not allow us to log on, either. I am planning on using Microsoft's Windows Defender Offline Beta tool (bootable CD is already created), to scan the PC; however, I am curious to know where else I should start looking.

Thanks for everyone’s assistance.

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