Hi, depending on the windows version you are using, you could boot into safe mode by pressing the "F8" key a few times while the computer first turns on.  It will give you a menu of choices for almost all windows versions.  For XP sometimes it shows a list of windows installs and you must select the "Windows Advanced Options Menu" then first try the "Last Known Good Configuration" option first.  If it is successful and boots up, then it should boot normally next time.  If that doesn't work try the "Safe Mode" option, use this option for any other version of windows as well.  This will allow you to scan for viruses, check system files and back up your data if you need.  Good luck.