I need help in getting rid of the virus or adware in my computer!?

My computer is infected of some sort, i dont know if its a virus or adware. Okay heres the problem, i have windows xp and when i start the computer, log in i only see my background image, no icons no nothing. Then i ctrl, alt and delt and click controlpanel and magically the desktop reappears (have to do this each time). Then once it appears o click safari and wont load until 5 minutes later. All of a sudden a get a warning message saying " application cannot be executed. The file safari.exe is infected. Do you want to activate your antivirus software now?", i click yes and it opems up an internet page that says i have to pay 69.95 using credit card. Not only is it open this but its opening ads for and viagra back to back pop ups. I try clicking on other things but it says the same message file infected. I have mcafee but my subscription expire. How do i go about getting rid of this manually?