Why is cpu at 100% all the time?

i have a dell latitude laptop which is about 2 years old. i recently formatted the HDD and installed Windows 7 64-bit version.it had vista basic when i got it.it has core 2 duo processor with 3GB of RAM.it's a good config for work.i don't have that much installed on it.the only software that run at startup is Microsoft's antivirus and the Intel graphics controller panel.yet my cpu usage is always at 100%.it makes it impossible to sometimes work on the laptop since it can get slow really bad.even when i'm running just one program namely google chrome,the laptop becomes terribly slow.i checked the processes running and the amount of cpu they each consume.bt none of the processes ever go over 40-50% except when i'm playing a video on WMP.even then it uses nt more than 80% for a few moments.i have tried everything.cleanign the registry, deleting temporary files and running virus scans and defragmentation.nothing seems to help.