Looks like its a very strange issue,normally it doesnt happen with the updates..
Try the following troubleshootin steps
1.Restart your computer in safe mode
2.Login to default windows xp administrator account
3.Check whether your old user account exists in documents & settings
4.Create a new admin account from control panel->user accounts
5.Restart the computer in normal mode & check whether its showing up ur newly created user account .811151
If it is showing up then probably your user account is corrupted hence to copy the data from old corrupted user to account to new user account follow steps mentioned in KB 811151..
If it is not showing up then try system restore from safe mode to an older date when your computer was working fine..
Good Luck
Hope it works!!!!
confidence