power supply filter

as I use my computer PSU to power other devices such as the monitor and a battery charger and also bearing in ming that HDDs cause electrical disturbance because they have motors I wanted to put some low pass filters between the PSU and these devices so as to keep the power to the motherboard as smooth as possible. using a LC filter what are the best values ? we are talking like 1-2 amps per device

You need to know the frequency the PSU runs at, and if it's constant or not. However, there seems little need for any additional filtering?, PC's and HDD's work perfectly together already!.

They do indeed work fine together without filtering. No doubt the filtering is included in the HDD and on the motherboard to avoid any potential problems.

I thought it as more of knowing the frequenzy of the device.

the reasons I wish to do this are that I use other devices other than the pc components for example I removed the monitors power supply and connected it straight to the computers power so turning that on and off can creat spikes. I also wan to use the same power supplt for ther things like my battery charger.

the second reason is that I overclock my computer so want as mauch stability as possible as I do not have dual 12 V rails

As long as the power dissipation of all the loads on the power supply is less than the rateing of the power supply no additional filtering should be neccasary. If you're really concerned simply purchase a higher quality power supply or one with a significantly higher than needed power rateing. A power supply under 3/4's of it's max load should have exceptionally good regulating charactoristics.

If you use an L-C filter you need to know the frequency and current characteristics of the load, but if you just add a low ESR capacitor near the load, the value is not that important. Use an electrolytic and ceramic in parallel, as large is is convenient.