Eventually, I reinstalled my system. This is my first time reinstalling Windows XP on my ASUS W3V notebook. For the last 4 years, it worked like a charm, although I installed a lot of programs on it, tweaking the UI for several times, and used it almost every day at work. Frankly speaking, it's rather robust and stable for me. The reason why I need to reinstall it is that recently the Window software installation system is broken on the machine. I can't remove installed applications, nor install new softwares. That bothers me a lot since I am kind of mania about installing new stuffs.
The installation of my new system took me some time by using recovery dics from ASUS. To my surpirse, it uses up to almost 13GB after the installation. I am not sure why it takes so much space. Anyway, everything works now again. Let's see how long I can keep it alive this time.