depending on the infection, what might have happened is that when the virus was zapped, the infected system file(s) were also zapped, which may have caused your system to become unstable. the problem with most anti virus programs is that they can zap the infection but cannot restore the original files before they were infection.
the xp disk can help you restore the corrupted or missing system files. however, the xp version on the computer must be identical to the version on the xp set up cd. so in order to execute a repair installation, you would have to downgrade your system and uninstall the sp updates from the computer.
but on the other hand, there is a method to restore system files via system restore from the disk prompt.
what i would do is to boot up with the xp cd and initialize the recovery / repair console in order to get you to the disk prompt.
afterwards, i would execute a old system restore point which should re-instate a stable registry hive and copies of system files before the infection occurred.
personally i would choose the oldest point, which would guarantee that the infection did not exist at the time. but the restore point is your call. in any case, the instructions can be found here:
but before doing the above, do a quick check disk to ensure that the master file table and the file system and index's are in sync and not corrupted.
if after you do a system restore and the machine is bootable again, boot into safe mode and disconnect the internet cord from the machine and also execute a clean boot. "or" you can choose to uninstall the sp updates via control panel, if you prefer to execute a repair install with your xp disk.
in any case, if you decide to boot back into normal mode, be sure to disable all browser extensions and add-ons and delete all cookies before reconnecting the internet line, as they may have also been the entry point of the infection.
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