coputer going crazy!

First I'm gonna warn you, i am completely stupid to computers. When i start the computer a message reads ERROR SOME STUFF EXE. its different every time. Also limewire opens itself even after i close it rapeatedly. THis happens continously until i just leave it open, even then it will sometimes close itself and then open up again. Also tons of pop ups and web sites just randomly pop up. I'm having a difficult time typing this due to the constant interference by the pop ups. Ive scanned my computer with the usaull norton antivirus and spy ware. although i do recieve messages about my registry when i run nortons. NOTE: none of this started happening until i tried to download a flight sim program from limewire. Know its outa control, please help. my computer is a 2003 compaq, pentium 3, 40G, cable modem,windows ME,