Noise problems

Hello everybody,

Its a pleasure to be here, and my incentive to join here is of course a technical problem which I don't know how to resolve.

I am dislectic and pretty ignorat about my facilities but I will give it a try.
I'm working with:
- A behringer mixer (aEurorack UB 802)
- Two M1 active Alesis monitors
- An M Audio 2496 sound card
- Cubase (PC)

I have high quality cables, all new, and I am very happy with my current monitors. They are definately the best you can find in their price (100 euro's a piece).

Now to my headaches:
I have different noise problems which make it almost impossible to record here. I recorded three full albums on this equipment in my former home and I didn't have these problems then.

Here is a list of tests I made which are not consequent enough for me to conclude anything particular:

1) When I record a guitar through the mixer (direct in jack) through a guitar rig on cubase, I receive an annyoing white noise in the background. I play double and single coil. When I switch on a distortion effect it becomes impossible to record. I tried directing the guitar straight to the interface, which cleaned up a lot of noise, but I feel it can be in more acceptable levels. There is a constant F# tone (1.5 octaves above middle C) which is humming in the background.
I then disconnected the Alesis jacks from the mixer and the F# tone was gone.
I can also stop the noise by closing down the cubase application, but thats not very handy for me.
My question: How can I keep using my monitors eliminating the noise in a different way?

2) I keep hearing a constant repeating buzz from my left monitor. He is situated above the computer, at the same distance from the screen as is my right monitor. Its very soft but realy unnecessary.
Any tips on what might cause it?

Thanks a million for your help. Any tip would be welcome, but detailed proffessional help will definately be appreciated!

Thanks and a happy new year!