Hi,
Try cleaning the slot and the contacts on the bottom of the GPU, with a soft paint brush
Also clean the fan on the card as best you can.
Artifacts on the screen are normally due to the RAM on the GPU being too hot, but as this happens straight away I doubt its that...
Make sure that if you have onboard GPU that you disable it in the BIOS.
Insert the new card and then, go to the Nvidia site and re download the correct drivers, remember that you need the EXACT drivers for your card and the OS (both version and 32 or 64 bit)
Then go to add/remove programs in control panel and remove the old display driver, DONT restart, got to C:/Nvidia and DELETE any folders within that then restart, then install the new drivers, then restart - all 'should' be ok then unless there is an issue with the card itself which will be apparent after you have cleaned it all up :)