Chances are your Master Boot Record is now corrupt giving you a looping system.I think a repair is best here click here
Alternatively you maybe lucky if the computer has a recovery partition.
Now im unsure if it has the restore/recovery partition on Fujitsu Seimens but try tapping F11 or F10 on startup,this may take a few attempts.This assuming it does have the partition.
If this exists,you will be welcomed by three options normally;
1) Repair xp with data loss
2) Reinstall windows with data loss
3)Restore to factory settings (destructive) everything will be lost and back to how the machine was bought.