Dell :: Slowness In A 2 Years Old Vostro1500, Win Xp Sp3

I have a vostro 1500 (T7300, 2GBx1 ram, nvidia 8600m GT gfx). It's about 2 years old and warranty just finished. OS is win xp sp3.

Recently I feel the computer becames really slow, especially after I opened BT downloader. I first thougth it is hdd problem, so I swapped the original seagate 160GB with a new WD 500GB, OS and all software cloned to the new drive using seagate's diskwizard. I didn't do a clean install because I have quite a few software that is troublesome to re-install and re-configure. The problem didn't get solved. I then checked the hard disk with seagate tool, no problem found. Did de-fragmentation, no improvement. checked the DMA mode and it's normal. I even opened up the laptop to clean the dust in vent and noticed a temparature drop when full loaded from 86C to 75C. But that doesn't solve the slowness problem.

So up to this point, I think maybe I have to do a fresh install. But before I do that, I hope someone can help me with other alternatives, to identify the problem or solve it.

the symptoms are like: slow startup, slow reaction to almost all actions, like opening browser, opening windows explorer and folders, opening applications. Once it is opened, performance is normal. I also tried copy large size and quantity of files from the hard drive to external hard drives and vice versa. For large size files, speed is normal, but for many small sized files, speed is terrible. It looks like disk buffer is not enabled or sth, but I don't know how to check that. After all, the new drive I just switched to shall be a good one because I used it on another laptop and it's fine.