Does having the Bitcoin Client running always use up a lot of the CPU?
The normal bitcoin client should not use more than a few percentages, if not even zero, from your CPU.

What client are you running? The Bitcoin-Qt client? Did you ever enabled the "generate bitcoins" option? I'm not sure if it's in the Bitcoin-Qt client's settings, but you could've enabled it in your bitcoin.conf file.

Maybe you should post your bitcoin.conf file. Or just make sure it doesn't have generate=true in it.