If you can start in safe mode then you probably have some corruption in windows. You can try running sfc /scannow by going to Start and the run and type that command in. You will need the windows xp cd in the cd rom drive. If this fails then you can try booting from the windows xp cd and selecting repair install. By default the is no password on the administrator account. If it lets you in then run chkdsk /r and see if that fixes it. If there is a password, then press enter to install windows at the first screen and then install windows into the same directory that it is in now and it will be overwritten (repair install). If this all fails then a complete os reinstall may be needed