|"i recently reformatted windows xp because i didnt feel like spending money to get a deadly virus removed"|
In other words, you didn't know how to remove the virus yourself.
"everything was installed properly"
What does that mean? Hardware was installed properly? Windows was reinstalled properly?
"the video card worked at first at its lowest possible settings"
In other words, it works using generic XP video drivers.
"after i inserted my catalyst CD supplied by ATI and installed the drivers, it stopped working"
Meaning what? You get no display at all? Nothing on the screen even at bootup? You need to explain yourself better.
"i have an ati radeon 2400 HD PCI"
PCI or PCI-express? If it is PCI, does that mean your board has no AGP or PCI-e slots? Is this an OEM system (Dell, Gateway, HP, etc) by any chance? If it is, post the model number.
"I also forgot to unhook the new card before i reformatted and do it with the old one"
It makes no difference which card is installed when you format the HDD. Everything is being erased & you're starting with a clean slate, so there's no chance the new card drivers would conflict with the old card drivers.