I have windows xp pro sp3 in sony vaio desktop. Computer has been slow for a week or so and suddenly it crashed and now shows black screen with blinking cursor.

I booted the computer using hirens boot cd with "boot from harddisk" option and restored to previous dates. But still the same does not boot on it's own. But I can boot through boot cd and continue my work, only thing is it is very slow.

Any idea what should I be doing to figure out why it is not booting through hard drive and why it is so slow. Thanks!

To me this usually indicates a hardware issue. The first two things I would do is check hard drive and memory for errors. Hirens should have utilities for both.