Windows 7 CPU 20% after idle

[dapilot400]

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.

 

[Highwayman]

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

 

[dapilot400]

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

 

[reghakr]

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?