Speakers buzzing!!!

Hey peeps,

Having a serious buzzing problem from my PC speakers. Im using a Mercury 5.1 surround system bought about 4 years ago. Its been perfect all this time until the last few weeks when a buzzing noise would come and go.

Lately its a constant noise unless the speakers are powered off completey. I unplugged any connections to my pc to make sure it wasnt a grounding problem or error from my soundcard but it made no difference.

I've gone through all the speaker cables to make sure none are faulty but cant find any.

Anyone recommend any solutions or replacements if these are fubar!!

I use my pc as an all rounder, gaming, movies and music/music creation so i need a decent set on a budget of about $150.

Thanks all!!!

My rig -

Vista Ultimate
AMD Athlon 64 X2 5000+
ASrock ALiveNF6G - DVI VSTA
4GB Kingston DD2
ATi Radeon X1950 XTX
SB X-FI Fatality
Dual SATA Maxtor 250GB
Dual IDE Maxtor 150GB
LG Lightscribe DVDRW/DVDRAM/DL
LaCie 22'
Wizard Case (650W)