Best bet like most Nvidia driver when the chipset is no longer supported is to go back to a driver version when the video card was actually supported. Can't remember what it was for TNT2 series but probably somewhere in the 50x series.

I would make sure that Chipset and GART drivers are properly installed. Look for those drivers that are specific for that motherboard.

May be worth trying a drive cleaner utility to make sure you get rid of all the old driver files before installing a new one.

It's a stab in the dark, but a few passes with memtest wouldn't hurt.

Tried another card yet? That graphics card isn't particularly good for your system anyway. I use cards like that on sub 1Ghz systems myself.

Other then that, what drivers have you tried so far?
This thread suggests some versions for your card on XP
http://www.nvnews.net/vbulletin/showthread.php?t=28917

I have installed both the nVidia driver and SiS AGP Port driver. I also tried reinstalling the Windows, no luck yet.

About the memory, I just ran MemTest86+ (the one shipped with ubuntu) for 2 times, and no bad memory blocks. The memory itself is a new memory (around 5 months). The old memory has some bad blocks and I exchanged the memory with the new one, as it has lifetime warranty. The memory is 2x 512 + 1x 128.

About new card, I will get a hand-me-down ATi card with 128M memory in a few weeks. It has several issues with Windows 7 not appearing correctly, but as I am not using Windows 7, it will be okay for me. This card is shipped with me when I build my computer (as I remember, I got it bundled with motherboard or got it as an extra, because I didn't paid for the card).

You should try an older NVIDIA driver as suggested by DOSFreak. I've seen the newer drivers be unstable with TNT and old GeForce cards.

I'd go back to a 45.xx or 5x.xx pack.

Be sure to uninstall the current driver before installing a different one.

I uninstalled the driver (at present is using Windows generic video adaptor) and then reinstalled the driver using the driver from CD (it is 45 point something, I don't really remember as I am in Linux now). It is generally more stable. But I lost the nVidia control panel, it doesn't matter as I never used it.

It seems that the culprit was I am installing the driver on top of Windows generic driver for nVidia Riva (which is for a broad range of card, Riva / TNT / TNT2 m64). It is fine now.

Thank you for all your support and suggestions. This thread is now solved.