i am using Crux with kernel 2.6.20.3 and whenever i try to compile something (for exemple) my computer starts having a strange behavior, meaning i am not able to do other tasks, xmms stops playing and things like that. intensive cpu usage tasks totally block the system while they are working...maybe i need to change something in my kernel configs?

where is the link with my config: Thanks

My guess is that it's because of the IO scheduler.
You have
You're running anticipatory, which can cause large delays, probably like the ones you have. Try a different one, for example deadline scheduler.

Another thing to try is:
And make sure you have dma turned on on your disks with hdparm.

For the record, I switched TO Anticipatory, FROM Deadline, for a welcome improvement in performance..

The reality is that you can guess all day long and that's all you will be doing: guessing!

If the system drops into a hard-freeze of the sort that can kill a music-player, it's probably a configuration error of some kind; you typo'd something.

Roll back to the previous kernel, and I hope that you kept a backup copy of the various /usr/src/linux/.config files that you've been using! If you did, you can simply use diff to see what you did wrong.

Also, take a close look at the system logs, e.g. /var/log/messages. Usually, there is a clue there.

I don't know if you compile your own kernels or not, but if you do, it's important to make sure that the compiles are known clean, in other words, to be certain that everything did get recompiled and that there's nothing "left over."