boot into safe mode, unistall the ati drivers, reboot normally. if the problem still persist, reboot into safe mode again, then install the nvidia drivers
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E8400 3Ghz || 4GB OCZ 1066 DDR2 RAM || GIGABYTE GA-EP45-UD3P Motherboard ||
HD 4870 512MB || Raptor 300GB 10,000 RPM HDD (
150GB X2 @ RAID 0) ||
Windows 7 Professional 64-bit