Ok, updated now:
Did what Herron told me - no help
Run few more antivirus programs - nothing founded
So, now I believe Jitspoe was right - it's probably some hardware issue.
Now I'm sorry but i seek further help because I'm clueless what causes this lag. I cleaned up my comp with ccleaner.
I'll try to describe the lagg - during the play, every few seconds my picture freezes, can't move, than I found myself on a totally different place. Funny thing is: when I replayed the demo i uploaded here, it lagged, but my other demoes don't lagg.
Any idea?
Edit: other players can't see me lagging