CPU 20% after idle

After leaving the PC idle for a few hours, the CPU usage increases to 20%. This is a very large increase from the CPU usage after starting up (and a few hour later while using the PC) were the usage is approximately 3%.

Notes:
CPU: Core I7 920 (2.8 quad core)
Mother board: MSI X58 Platinum
3GB Ram
Win 7: 64 bit, clean install
System and system interupts are the biggest chunks (~9% and ~5% respectively)
Downloads: Firefox, MSN messenger, yahoo messenger, Avira antivirus, Ad-aware , Winrar 64 bit (drop was occurring before avira, adaware and winrar downloads)
Scan with avira and ad-adaware come out clean
despite the high CPU usage, computer still runs smooth, have not attempted with any games though.

i's suspect defrag on shedule, or maybe antivirus scan on shedule is the culprit

Well I will attempt a defrag, but the first thing i did was scan with Avira and ad-aware and both were clean.

Have you checked the processes in task manager and see which is using the most?

System and system interupts are the biggest chunks (~9% and ~5% respectively)

what would thos be?