It is very likely that a virus has infected your system. If you haven't already, run some virus/spyware scans.
Without a recovery disc or Windows XP installation disc, there may not be much you can do, but try these:
1. After you turn on the computer, keep hitting the F8 key. This will bring up an advanced menu.
From this menu you'll be able to select several options. Use the Last known good configuration option.
2. If you can get into Windows, there is an option for a System Restore. This will take Windows back to a system checkpoint that was made in the past. This can help with your situation, but there is no guarantee that this will work either.