Non-alarm and recurrence problems

I haven’t had this problem in a little while, but it seems to occur a bit, so.
What can I do and why does it happen when a virus is detected and I press delete but the warning comes up again or doesn’t get deleted? And the other question is… If I have a program that I COMPLETELY KNOW is a virus, or is not a valid program, is there a way I can make it to be found as a virus?
Because I just pretty much activated this laptop I’m on and I forgot to put Avast! back on it.
I download Firefox and start to use it a bit… I get this program that comes up “Antivirus XP 2008”.
I 100% know its not a valid program because it downloads itself without me being asked or me going near their site. (If they have one)
It wouldn’t uninstall it self when I tried, so I used Moveonboot, and it was gone when I restarted.
That was yesterday, It came back today, I took it off again, same way.
Also, when it runs its virus scanner, it finds something over like 700 viruses with names like…
“system32\virus.exe”, or “windows\virus.exe”.
And Avast! only found like… maybe 4 viruses on full system scan, and I took them off…

Yes, I posted this in the “Avast! 4 Home/Pro” Section before, but I guess this is the right place for it, because its more about viruses.

I suggest:

SuperAntiSpyware Free
RogueRemover
MalwareByte’s Anti-Malware