sound card/speaker fault???

Hi. I am using a P4 2.8ghz with 2 gb ram and a gigabyte MB (cant remember the model. In have boston acoustics 5.1 speakers (which are quite a few years old now) and a creative SB Live value sound card(yes i know its old!!)
I have had the same problem a few years ago but for the life of me I cant remember how I fixed it.
I had to reinstall XP (SP3) a couple weeks ago and after doing so and installing all the drivers all was working well. About a week after the new install i was playing a game ( but it can happen anytime doing anything) and i lost all sound. Then a few minutes later I got this hissing test tone going from 1 speaker to the next. If I turn off the PC it still happens ( all speakers are powered from the sub). The only way i can stop it is to totally kill the power. After the first few times this happened I found that if I turned the volume down ( to get rid of the annoying noise) after a couple hours when i turned the volume back up all was good again. But over the last 3 days it has been getting much worse. Now it doesn't go away after a couple hours. If I kill the power and turn it back on after a couple hours its doing the same thing. When working properly I am able to make this test tone thing happen manually but i can also turn it off. I have reinstalled all drivers and checked device manager , all the no avail. I must say I'm stumped and HELP!!

 

do you have onboard sound that you can try out for a while? If it doesn't happen with the onboard sound, then you may have a problem with the sb card.

 

I do have on board sound but it wont run my surround speakers at all. I have previously had this problem but cant for the life of me remember how we fixed it. Though Im pretty sure I came here for help was a few years ago now though.
I mucked around uninstalling drivers etc today and managed to breifly get rid of the test tone hissing but still had no sound. When trying to get sound back after about 30 minutes of checking things i did manage to get the hissing back but still no audio.

 

OK. I thought I had a win yesterday. I opened up the PC tower and there was a lot of dust in it. I took it outside and blew it all out with compressed air (gently) and cleaned it right out. When i turned the PC back on after that I had sound again! everything was working right for about 2 hours! I had full audio out of all speakers including the sub!
Then I decided to watch a movie and as I opened my movie drive the test tone hissing started again! It doesn't seem to matter what I'm doing when it happens.
This is getting really frustrating.
I have tried uninstalling the realtek on-board sound drivers and disabling the on-board sound through the bios as had read in another forum that this might help but no difference. I have always had the realtek and SB live! installed at the same time and this has never been a problem.
I was really hopeful that the major dust problem was the answer.
HELP!!