Hi len
There are a few things to take a look at with your system if your computer locks up and won't boot.
For details follow the link
http://ask-leo.com/my_computer_locks_up_and_wont_boot_what_do_i_do.html
http://www.techrepublic.com/article/10-things-you-can-do-when-windows-xp-wont-boot/6031733