Maybe you need to remove the new card and if you can get into the system remove the drivers for the old one.
Or was it onboard video???
i recently purchased an nvidia mx400 video card for my sons computer since it lagged while he tried waching his dvd's on the computer. i've tried installing the card on windows xp, but everytime i boot up the computer the monitor just turns black. i know the card works because i installed it on my other system with windows 98 on it and it works normally.. so i just need some tips or ideas on how to make the card work on XP