I'm computer illiterate so please be patient with me.

I'm running Windows xp Home Edition

Two nights ago, i went to bed (computer running) and my computer was fine. I woke up to find nothing working. When I restarted my computer, my background changed from my normal wallpaper to a message similiar to this: "warning spyware detected on your computer install an antivirus or..." The weird thing is, it didn't show up in a window but as my wallpaper. When i changed it backto normal, it would simply revert back to that message when i closed the properties window. By running different adware/spyware/virus programs, i managed to delete that message. How do i know if i took care of the problem or simply got rid of a small part of it?

Also, i have my computer password protected. Normally, the only time that i have to use my password is when i turn my computer on. Since "that morning", i get a "run as" window every time i click on something forcing me to enter my password for every program.

I'm almost positive that i neglected to provide all of the information needed to help me. Please tell me what information you need and i'll do my best to provide it.