virus in my pc please help

hi i m using vista ultimate addition and having internet. i got a virus/spyware i dont know. but this one slow down my pc and disable my registery and task manager option. also it autopmaticly turn my pc UAC atomatticaly off and automatically turn “dont show hidden object” on….
please help i have reinstalled many time my window….but getting the same problem all time…i also used some antivirus like mac-fee,quick heel,and spyware like webroot..bt no improvemrnt..it doesnot allow to install avira antivirus. also does not allow me to install vista sp1.

please help please…i m in great trouble.