virus...
sorry mate... you downloaded a virus... windows is Hosed
unless,
put your XP disk in, and do a repair install when it asks if you wannt Run a repair install
note that its not the repair console you want.. continue as you would normallydo for a fresh install and it will detect windows already on there and ask if you wanna run a repair install
do the repair install
if this don't fix it... then... bubi windows