Question re: DPC Latency Tool results

 I've downloaded and run the DPC Latency testing program and have some serious problems. I have a Dell 620 Inspiron - which I'm sure is part of it. I usually build my own machines, but didn't have the time when my last machine finally died (long story). _____Anyway, running the test, I get from 1-6 sequential large red bars which run up into the 4000 range. The problem area seems to remain constant, but the number of bars varies as does their severity. Is there any way to determine specifically which those areas are or must I blindly go through Device Manager one entry at a time until I come across it by chance?______System specs: i3 CPU, 6GB RAM, W7 Home 64 bit, Focusrite Scarlett 8i6 USB interface, multiple hd's (most internal, one external via usb).

Speedstep disabled?

Some of the suggestions on the Focusrite site, while good, are not valid with SONAR, such as setting processor to "Background Services". For extreme DPC spikes there are several factors, normally wireless ports and adapters are huge culprits. Another biggie is as eikelbijter says, disable Speedstep if you have an INTEL CPU. That can be done in the BIOS. I had an issue with my system where Speedstep was ignored by Windows XP, when switching to Win 7, my DPC spikes were through the roof. Another BIOS tweak would be disabling the HPET function as well.

We have a winner! The Dell Wireless Controller was the culprit. Rerunning the DPC tool shows a very clean graph with an absolute (max) value of 158us and no spikes whatsoever. Speedstep doesn't seem to be having any effect one way or the other so I re-enabled it after initially disabling it for testing purposes. Thanks again, everybody!

Yes, wireless is one of the main culprits.  I forgot that the I620 had that (I should know I do support on the Dell users forum).