Help my virus!

Hi MajorGeeks :major, I have a virus on my computer that I got from a pop-up.

I have tried to remove it myself and I can make it go away but it comes back when I restart. I tried ending the process but the names are different every time, though its quite similar.

When I first start up the computer, I login then I get my desktop changed into a blue screen saying "WARNING VIRUS DETECTED. INSTALL A ANTIVIRUS TO REMOVE NOW." It's something similar to that. Then aVast says it detects a worm in my temp files. I delete it and blue screen of death comes.. it says my computre is restarting then it shows the windows xp loading screen but I am still on my desktop so I think its fake. I end the process and my computer is back to normal.

Whats the matter...