my internet speed changes

So.
i have elisa 24/10 but lately it can only download 100kb-400kb

and if i try watch in youtube video it cant play it even in 240p im clueless, it started about 2-9days ago i cant recall exactly time.

And im pretty sure its now virus because i have formated 2-9 day ago my computer and i runed virus scan 1day ago (norton full system) so i dont know what to do

Unless you did something to your computer that changed your internet connection settings, this problem might be with your service provider. You could try using a system restore point that goes back to a date/time prior to when this problem first started.

http://www.sevenforums.com/tutorials/700-system-restore.html

If a system restore doesn't help you can undo it if you want to.

System Restore - Undo - Vista Forums (same tutorial works with Windows 7)

Personally, I would contact customer support for your internet service provider. They should have the tools to run diagnostic checks on your computer and their own service.