I reckon your HDD is kaput. Here is the fix for your missing file/registry problem:
http://support.microsoft.com/kb/307545
Your previous problem sounded like your master boot record was corrupt. Both these issues point to a failing HDD.
I would consider purchasing a new HDD. Open up your PC and establish which type of HDD you have (EIDE or SATA). (picture here: http://www.aplusonsitenc.com/SATA-IDE.jpg)
Configure your existing HDD to be your slave/secondary drive and your new HDD as the master/primary drive. Install your Operating System onto the new HDD. You should then be able to transfer the files from your old HDD to the new one via your new Operating System.
Hope that helps,
TEk
