First thing, if you are using Laptop computers or branded computer you will have facility to lock your harddisk using master and user password. Reset your harddisk from Hardware portion.. from BIOS your won't get help !!
BOND - JAMES BOND
April 2008
As you don't say what the PC is its difficult to answer. However in any case howe am I supposed to know that it is your PC and not stolen? Take it to a computer shop with some ID and proof of ownership.
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Try fixfault.com to fix computer hardware.