You won't be able to recover the account, but you can possibly redirect Windows XP to use the old user profile. Before continuing with these suggestions, back up both user profile folders, as it could lead to problems. In the registry, change the path of the user profile folder that's associated with this user's SID. The profile list is in the key HKLMSOFTWAREMicrosoftWindows NTCurrentVersionProfileList. There's one subkey for each SID. Look for the user's new SID, and then change the value of ProfileImagePath. It's worth a try.
This was first published in June 2003