MacBook Pro running extremely slow!

Hello, recently my MacBook developed a multitude of problems foremost is that it runs extremely slow about 5 minutes after booting up. Activity monitor shows that CPU usage is 5% user, 95% system and 0% idle. The process or processes that are demanding this high system CPU usage are not shown in the list of processes. The result is that the MacBook runs extremely slow even the curser lags as I move it and opening any application takes at lest 5 minutes.

On top of this, it takes several attempts to boot the MacBook, it either lags early and the rotating icon below the apple symbol freezes and I am stuck with that screen or again it lags early takes sever minutes to load the grey login page and continues to lag as mentioned above. Also before my MacBook boots the light flashes five times (I have read that this is a battery problem).

My MacBook only works when it is plugged into the AC charger and constantly displays the green full charge light, removal of the AC charger causes the MacBook to turn off.

I have tried almost everything, including disk repairs and disk permissions repair, no errors reported. Also tried most boot command resets, to no avail.

I'm running OSX lion 10.7. HD is less than half full.

Please help. Thanks.