Lower the video settings:
change graphics to fast.
set smooth lighting to off.
3d anaglyph to off.
particles to minimal.
Render distance to tiny.
performance to max fps.
view bobbing to off.
clouds to off.
set vsync to off.
It may be because you are in a protected zone on a server or you could be in adventure mode. Sometimes there is also block lag which can even happen on singleplayer.
Your computer is old.
there is no viruses on minecraft just lag and pixelmon has none to
Yes, but it might lag.
That depends on your computer.
There is none. Lag is produced from your own computer, not recording systems.
If your copy of L4D lags in Singleplayer, then yes, TES:V Skyrim will lag profusely. If L4D lags in Multiplayer, then it just may be your connection to the internet.
Currently HD texture packs are not supported my Minecraft, and therefore are bound to lag. Having a powerful computer may prevent lag from occurring due to an HD pack, but not always. To fix lag due to HD packs try making sure the only program you are running on your computer is Minecraft as HD textures tend to make Minecraft require a lot more RAM.
use a 64-bit java to run minecraft. Or install optifine to reduce lag
maybe end of stream? (LAG)
Yes.
A slow/low end broadband speed is usually the #1 problem for lag on servers. The other is too little ram on the computer on single player, 4 gigs should be the minimum you should have. If you go to the minecraft WiKi they can tell you how to add more ram to the Minecraft application if you have plenty on the computer. Things that you can do to reduce the lag is to turn down some of the visual options in the menu.