Do you get that error message before you even try to log on with a user .. so during the boot of WinXP .. telling you one of the services failed or couldn't log on. . ?
You probably have a service that wants to start but because you restored your system that service is no longer actieve. Like a programm that automatically boots on startup. Only because of your system restore those registry keys are no longer avalable. . some thing like that. So you get the error that the programm / system couldn't start.
If not, did you already try to log in when you get that error message .. and with which user did you try to log on. . is it different from the one you can log on at a later point ?
Could you be more presice on the "as since yesterday morning I couldn't start windows" part .. as Windows needs to be booted [ upto some point ] before you can use the restore function .. .
Last option, if you log in as a user and after that your system automatically reboots. . . [ you are in deep . .

]. Could you try to log on as administrator and see if it still happends ?