If you are using Win95, or Win98, replace user.dat and system.dat with user.da0 and system.da0, then rename them back to the .dat extension. These files are located in the Windows directory.
If you are using Win98SE, replace user.dat and system.dat in the Windows directory with one of the backups located inside rb00x.cab located in the Windows\Sysbackup directory.
There are 5 backups located in that directory....rb005 is the latest, rb001 is the earliest of the 5 available.