FSX Lagging?

Hey guys. I have a pretty sweet PC, but when I try to play FSX on it, it lags every 2 seconds. I'm running it on medium high settings. My PC has 4Gb of RAM, an ATI radeon HD5570 1GB graphics card, a 3.04 GHZ quad core processor, and an MSI Motherboard. What's the problem? I tried running FSX on really low settings and it would still lag once every 2 seconds.

Thanks,
Eliot.

Standard installation?
Any add-ons?
Does this happen with other software? If so...
Do you have something like Google Desktop which could be indexing in the background?
Anti virus software running a check?
Any peer-to-peer programs like Blizzards updater for WoW running in the background?
Has your disk been defragmented lately?
Just some thoughts.

Seconding naflod on double checking that nothing is running in the background and that you've defragged since you installed FSX.

It's not a bad idea to turn off antivirus while running it, as long as you remember to turn it back on when you're done