Win2000 "random" freezes

[dhakir]

I've been having this problem for almost a year now.
My Windows 2000 had worked fine for a year or two, then someday it started freezing out of nowhere. At first, I thought they were random freezes, but since they occurred several times since then, I have been able to identify (most) situations which make it freeze. However, sometimes, under the same circumstances it works fine.
I've noticed that when I try to:
1) Read an e-mail with embebbed images using some program like Eudora or Thunderbird;
2) Open a Java applet;
3) Watch a movie;
4) Or using some DirectX programs (lesser chance of freezing),
my computer has a, let's say, 60% chance of freezing - by 'freezing' I mean: the mouse stops moving, the HD stops working, everything stays completely halted, until I manually reset it.
I've been looking for help in forums for quite some time now, but I have never found a problem description similar to mine. One of the most interesting things are the symptons that sometimes show up just before a potential crash: the HD starts working (I hear the noise), the HD activity light stays on for 2-3 silent seconds, and then it hangs most of the time; if not, the HD works noisily again and everything goes back to normal. Other interesting facts I found about it are:
1) It usually happens only once after I start Windows. I mean, if it survives the first potential crash, it is VERY unlikely it will crash again until I reboot;
2) If I happen to be fast enough and notice the pre-crash symptons, I desperately press Ctrl+Alt+Delete and stay in that "Windows Security" screen until the HD stops working heavily. This prevents the lock in almost all cases. Also, the computer has NEVER locked while that screen is showing up;
3) By combining the two facts above, I created a flawless method of watching movies: I use GSpot to test the movie (for installed codecs), press Ctrl+Alt+Delete, and wait for a few seconds. When I return, the computer has passed the "first time" test, so that I can watch whatever I want until my next reboot.
However, the above method does not work when I open an unexpectedly image-embebbed e-mail, or when a Java applet loads suddenly in my screen. In these cases, there's a high chance of freezing. If I miss the first seconds, even if I press the keys, sometimes it's too late, and the computer freezes before showing up the "Windows Security" screen.

I'm not sure, but I believe the problems started not much longer after I switched my original Rage ATI video card to a Nvidia TNT Riva. The memory sticks are original, and I believe there's no problem with them, as there seems to be no ventilation problems whatsoever.

I don't know what specifications could help. My computer is a Compaq Presario that came with Win98 installed, and many features got disabled when I upgraded to Win2k (including USB and onboard LAN - anyone has a hint on how to enable it?).

Any help is appreciated, but I really can't buy new memory sticks or switch video cards to test (the other one isn't working anymore). Also, my HD has had some bad clusters problem some years ago, during a voltage peak, but it's working ok since then, bad clusters isolated, and I believe it has little to do with it (though I'm not sure, since I can detect the symptons with it).

Thanks in advance,
Andre.