When i installing a game or zipping or do them both my cpu usage is only 25%-35%.
I have E6750 cpu, is it possible that it doesn't use its power becuase it get to hot?

it is probably becuse the app does not need that much usage. that is a very powerful CPU, you have to remember that how fast you can install a game aand such things will be limited by how fast your harddrive can write usually. A computer is only as fast as it's slowest component. I could be very wrong but if a cpu is processing something faster than another component can handle, what is the point? you will not get things done any faster.

Run folding@home that will get your processor doing something.

Strangestranger has it right. If there was such a thing as a 10Ghz Octo-core chip, it probably wouldn't zip everything at super fast speeds. Your limited to whichever components are being used. For installations and zipping your HD is used and is certainly the slowest component. It doesn't mean you should run out and buy the fastest hard drive you can, because you might only squeeze 2% more CPU out of it. 25-35 is just fine for zipping and such.

oh and as a general piece of advice, you should do a disk check every few months and a defrag every week or after a big installation. Windows will be able to install things faster if there are big chunks of continuous free space AFAIK.

If you have a standard hardrive rar A 3 GIG file if your cpu usage isn't 90% up you've got a problem

Has nothing to do with "doesn't use its power becuase it get to(o) hot". strangestranger has it right: "A computer is only as fast as it's slowest component". Which happens to be usually the harddrives.

One thing you might consider doing, if your data IS NOT critical, is doing a RAID 0 setup to increase performace. Or maybe investing in a Raptor. If you do go w/ RAID 0 setup I suggest getting into the habit of backing up any data on a regular interval.