I had the same issue on a Windows 7 Quad box running Oracle 5 in the VM.
Following Adam's advice i checked the Enabled IO APIC option but to no avail. So, i followed kmarsh's idea, which took probably less than a minute to try, and processor usage went down from 15-20% to 4-5%.
The settings used were, Name: Memory Hack, Type: Other, Version: DOS. Memory Size: 4MB, Hard Drive: Do not add a virtual hard drive. Click [Create]. Starting the VM asks for a start-up disk, i used: Host Drive 'D:', click [Start]. Virtual machine states: "FATAL: Could not read from the boot medium! System halted." At that point, the cpu usage dropped, so i minimized the window. Starting a second VM makes no difference.
ooooh it fixed it for me too! Awesome, thanks. For info, I'm running windows 8.1, VM is centos and was consumming at idle time around 15% cpu. VBox v4.3.12 – Sebas – 2015-03-08T23:22:56.307
Amazing this is still an issue. – kmarsh – 2015-03-09T14:17:25.310
nice to know, that vbox allocates possibliy used cpu-sources on startup – Diskilla – 2010-04-16T12:06:10.367