Sounds as if you know what you're doing.
Laptop hardware sounds fine.
It's Windows that won't load so probably due to a Windows update.
Assume you've tried the Microsoft suggestions
http://support.microsoft.com/kb/314503
First off, I'd wap out the hard disc and back up all the data.
In an ideal world you then restore to previous configuration save of OS.
http://www.howtogeek.com/howto/windows/using-system-restore-in-xp/
Otherwise, try to repair Windows with system repair/recovery disc.
http://www.michaelstevenstech.com/XPrepairinstall.htm
If all else fails, restore to factory settings (make sure you have a backup of all data).
Laptop manual should give you details of restore functions.