Need some computer help.

[Squarepants]

I need some help BADLY!
If anyone can help me that would appreciated.
I'll explain now.
It started a week ago....
A random window opened up called "Personal anti-virus system" , saying that i had some viruses and that i'll have to buy there anti virus system to remove them. I have AVG anti virus system and this thing is pissing me off.
I don't know how to remove it from my computer , when i go to control panel and go remove or add programs but it's not there and yeah. It's just bugging me and i need some help. If anyone knows anything that can help just tell me.
Continuing from up there ^^ , My computer is fine and i don't think i have any viruses i done a full computer scan and removed all "Threats".They are just annoying me trying to get me to buy there anti virus system.Yeah i just need help removing it.

Thank you.

[Arsenal]

It's spyware/malware. Never trust anything that pops up claiming your computer is infected. I'd recommend downloading Malware Bytes - Anti Malware and running a full scan with that. Make sure that its database is up to date otherwise it will fail to remove anything. If that does fail, run a program called 'Hijack This' and post the report log onto their forums. An 'expert' should be able to help you. And if that all doesn't work, run a program called 'ComboFix' and that should work.

Try all of them until one works. You can also post your 'Hijack This' log here and I can look at it. When posting it, use the code tags. :D
If anymore problems occur, post back.

www.malwarebytes.org
http://www.hijackthis.de/en

P.S: Don't google 'ComboFix', go to the Malware Bytes - Anti Malware forums and look for the download there.

[Squarepants]

Thank you very much arsenal! :D:D:D:D !!
It's all fixed now..
Tyvm.