odd cpu spike..cleared with reboot

for some inexplicable reason..my cpu usage spiked and stayed consistently at 25% tonight.. I identified the offending process as 'system' under windows processes and ran a malware scan and virus scan..clean across the board. I'm rather confused..as all I have open is vlc media player, skype, yahoo and chrome, not really doing anything that could be remotely considered cpu intensive. did one reboot that made it go away and then scanned my system...clean as stated above.

what I was doing at the time was chatting with a friend(on skype) and listening to transformers revenge of the fallen(score), and I had just unpaused the music..wasnt expecting the audio to stutter, which is when I pulled task manager up and noted the unusual cpu usage.

not really sure where to put this..so its under general.