Thanks to Igor for spending an hour on MSN helping me analyzing the reason of the poor performances of laptop.

He told me about a thing I was not aware of: the S.M.A.R.T. attributes for an hard disk. These attributes are stored inside the disk controller and should contain information useful to predict failures.

One of the most critical attribute is the "Reallocated Sectors Count":

Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks this sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping and "reallocated" sectors are called remaps. This is why, on modern hard disks, "bad blocks" cannot be found while testing the surface — all bad blocks are hidden in reallocated sectors. However, the more sectors that are reallocated, the more read/write speed will decrease.

Source: Known S.M.A.R.T. attributes

There are a few diagnostic tools that are able to read that attributes, and I tried the following one:

• SpeedFan: monitors fan speed and temperatures, and read SMART information as well
• HD Tune: this is a diagnostic tool for Hard Disk. Compute disk speed, surface damage, and read the SMART tags.

They both warned me that my HDD has 548 reallocated sectors (and usually disks have from 256 up to 1024 spare sectors), so I might want to change my disk before it's too late.

And that is reason for the poor performances of my disk... so I finally decided to buy a new 2.5" HDD, and replace mine, and wait a few months before buying the MacBook. So next weekend is reinstallation day, again.