Thanks, karlehenry, for your suggestions. As you mentioned, it was perhaps "blue screening" due to some corrupted system files . So I just reinstalled WinXP and chose the "Repair" option . Fortunately, it worked in this case ! I am back to normalcy now .
I was a bit surprised when you mentioned "reformat" the HD . This is a drastic and last resort solution only. Perhaps you have a reason . Please elaborate.