Hi sapna,
Excellent news.
I'm glad that the BartPE full sector scan worked. Because the next thing to try would have been manually replacing the corrupted registry hives with a backup from a previous restore point, using the BartPE disk, which is quite a complicated procedure if you are not familiar with it. But we don't have to do that now, so that's OK.
However, the fact that the three stage BartPE chkdsk DID NOT work, and the five stage full sector scan BartPE chkdsk DID work, means that your hard drive may have some 'bad sectors' on it. This could mean that your hard drive may be starting to fail. So I would recommend making and keeping backups of any important data that you store on this hard drive. But, saying that, I have also seen hard drives with 'bad sectors' keep working for several years. So you may be OK for a while yet.
To keep your hard drive in the best possible condition, I would recommend that you run a five stage full sector scan chkdsk from within Windows XP about once a week.
To do this, first click on 'start' and then click on 'My Computer'.
Then RIGHT click on your (C:) drive and click on 'Properties'.
Then click on the 'Tools' tab and under 'Error-checking' click 'Check Now'.
You will see a gui with two boxes in it, one which says 'Automatically fix file system errors' and on which says 'Scan for and attempt recovery of bad sectors'.
Tick BOTH of these boxes and then click 'Start'.
You will then see a box which asks you if you want to schedule the disk check to occur the next time you restart Windows.
Click 'Yes' and restart your computer to run the full sector disk check.
Regarding your USB problem, the Admins on this forum like to keep separate topics in separate posts. So if you post another topic about your USB problem, someone (maybe me) should help you with it soon.
Happy computing.
All the best,
~Mark.