You could re-install XP, but you should do a clean install if you are going to go that route. On the other hand ...
I am presuming you want to use the TNT rather than the ATI card. If you are getting nothing on screen with the TNT, put the ATI card back in. If the machine comes up allowing you to see the screen, do the following
Right click on My Computer, Choose properties, Device Manager, Hardware Tab. Click on the (+) next to the Display Adaptor, Right click on whatever Adaptor it sees. Choose Properties, Drivers Tab, Update Drivers. Then, choose the Standard VGA driver. Install it. It will want to reboot. Don't. Go to Settings, Control Panel, Add/Remove Programs, and Remove any ATI software that is installed. Reboot.
Let it come back up to make sure that the Standard VGA is installed. The screen should look crummy.
Power down, replace the ATI card with the TNT card. Turn the machine back on. If XP recognizes that there is a new card, it will try to install the drivers. If it does not, go through the same procedure as before Right click on My Computer ... but choose the driver that approximates the TNT.
XP and older cards are not a perfect match. After installation the drivers may work for windows but not OpenGl games. You may have to scour the Net to find drivers that will work both under windows and OpenGL

Thnx Sampson,
see when i tried to re-install Xp and i put the bootdisks in, it show nothing even tho the a:\ drive is reading. I had to reserve C800-CBFF memory adress range in my BIOS so my graphics card would boot properly.
when i did THAT an i retried to get XP it showed nothing. it would seem its XP that needs that memory adress or another of my hardware components is there anyway of finding out how?
I have put the old card back in and made the memory range available XP booted fine! i even tried doing that with the TNT2 card and without reserving the memory range that semed to work until i got the error message during post: Allocation static node error #00
in my mobo manual it sais to reserve that memory adress so that error goes away, but when i do that XP messes up!
Ive sent a help Message to Microsoft and they are gonna try and solve it too.

Every motherboard differs. The memory space you are reserving is typical for most graphic cards. Nvidia, 3dfx, and STB cards usually like to assign their cards to their own IRQ especially if they are PCI cards not AGP. Sometimes motherboards do this, sometimes you have to do it from bios. If it is an AGP card it is usually automatic.
 
Anyway, if you get XP up using your ATI card, you should be able to change the drivers for it from ATI to Standard VGA (see above). After rebooting the card will work, but it will not be using the specific ATI drivers. Many ATI installations have a little program running and there is a little icon in the task tray. You will want to remove that program by going to Add/Remove programs from your Control Panel.
 
Once the machine is using the Standard VGA drivers, you should be able to power down and replace the ATI card with the TNT card. XP will still be using Standard VGA and the TNT should work with those drivers so there should be no memory conflicts. Then, you can install the appropriate Nvidia software and drivers for the TNT once the system comes up.

One Question,
i tried to put the VGA drivers in but XP only shows me the ATi drivers, it wont show me the norm VGA drivers even when i uncheck the "show compatible drivers only" box, so how i get the VGA drivers!

You don't really need to - I've never used it. What I do when installing new drivers for my Radeon [you're supposed to uninstall the old ones 1st] is uninstall them 1st, then install the new ones with out rebooting, then reboot. I've found it works ok for other hardware in XP as well.
 
So in your case, you could uninstall the driver for the card that was in the system originally, shutdown & power off, put the new card in & power up. When Windows boots up it should see that there's a new card & do it's found new hardware bit @ which stage it may well install MS's drivers for the card [will most likely ask for the Windows CD unless U keep the install files on the HDD]. I'm guessing @ that bit as I always install from HDD [faster than installing from CD, plus saves you having to hunt around for the CD if Windows ever asks for it], but if it does ask you for the windows CD or the driver CD I would assume you could just cancel it, then run the install prog for the drivers. If you do have it set up like me [install files copied onto HDD] then wait til [if] it installs the MS v. of the drivers, then uninstall them [without rebooting], then install the drivers you want to use & reboot. Job done [hopefully ].

Thnx alien but ive already tried that and it wont work becuase windows wont boot. It seems windows cant decide what to do and just stalls when it should bootup! Ive sent Microsoft and e-mail since i got a support contract with them, so hopefully they can wokr it out. Ive tried all your methods but none have worked so far .
Hopefully either someone else or Microsoft can come up with an answer!