Windows startup and hard disk problems.

I have a WD internal hard disk.
i installed win7 professional 64 bit on 2gb ram system.

All problems started when there was date n time reset issue.
So i changed the CMOS battery.
Just after that,my computer started experiencing startup problems.
There was blue screen errors too ... for one or two times.
Many times i got the DISK READ ERROR on startup; but i was able to fix it up using the bootrec commands using the installation disk.

Some said that win7 64 bit should have atleast 4gb ram, so I installed the win7 professional 32bit back once again.
But the startup problems didn't stopped.

I was suggested that i should delete all my partition during installation and install windows on only one partition and after that i can make other partitions.
I did so. I installed windows 7 professional 32 bit and also made some partitions after installations.
Everything was going fine, but some problems are still there.
As soon as i started to load some things on my hard disk, my computer was slowly started to be un responsive.
I only installed some programs.
I just downloaded a 10GB material on my pc..and the computer started to lag.
I wanted to copy that thing on my mobile, but the copying was struck and it never happened.
Last night once again while watching a video,i got a blue screen error and after that my pc once again had the same old problem of DISK READ ERROR,PRESS CTRL+ALT+DEL to restart.
But my PC started today very fine, but lagginess is still there.
My PC is around 5 years old and so is the hard disk.
I also ran some tests on hard disk, i.e. CHKDSK from CMD, and all tests by SEAGATE TOOLS.
CHKDSK found some errors and corrected but all tests were passed by both tools.

So i just wanted to know, where the main problem lies.
Is it with my hard disk ? Do i have to replace it ? OR there is an another problem ?
PLEASE HELP ME ASAP.