Try setting the processor scheduling to adjust for best performance of background services rather than programs.
In Vista that setting is in:
Control Panel >> System >> Advanced system settings
That gets you to the Advanced tab of the System Properties dialog
Click on Settings... under Performance
Go to the Advanced tab and select "Background services"
Try that and see if it helps.
There is a general guide for optimizing a computer for digital recording
Optimizing Your System, Improving Performance and Managing Latency
It's more oriented to optimizing a computer used in a studio. But some of the tips will still apply.