Snow Leopard becoming very slow

I bought a 17 Macbook pro four months ago. It was fine the first days. But it has become very very slow and hard drive makes a lot of noise.
It takes like 10 seconds for spotlight to find stuffs.
I used Disk Utility and a 3d party app to make sure there's nothing wrong with the hard drive.

Is there a way to fix hard drive slowness without having to format the Snow Leopard?
If I use Time Machine to back up Snow Leopard on an external hard drive, then format Macbook pro's hard drive, and then bring back files to it using external the hard drive, is it going to have any effect?

I did a Repair disk and it helped a little. However spotlight is still almost useless.
I really like to know the answer to this question:
If I use Time Machine to back up Snow Leopard on an external hard drive, then format Macbook pro's hard drive, and then bring back files to it using external hard drive, is it going to have any effect?

Thanks a lot Hellhammer. I'll back up everything and reformat the hard drive.
There's about 150GB free space out of 500GB.

celticpride678 said:

It might be a little late now, but do you have a Boot Camp install?

Click to expand...

No it wasnt late. Turns out Boot Camp caused the problem.
Last night we got a brand new 27 iMac and after installing Windows 7 all of sudden Spotlight went crazy.

Since Windows 7's partition is NTFS Spotlight gets confused.
I added Boot Camp partition to "prevent spotlight fom searching these locations" in system preferences and that solved the problem.

Doing the same in MacBook Pro solved the slowness problem.
Thanks