Strange sounds going on

So I installed iTunes about 1.5 weeks ago on my laptop and it crashed importing my files (rubbish but not really the point). Ever since then my sound will randomly either be replaced by nothing, by a loud (white noise-type) hiss or a combination of both the file I'm trying to play. As far as I can tell it started as soon as I installed iTunes. Since then I have uninstalled iTunes and that didn't help so I even went to a restore point like 2 days before I installed iTunes and still it happens.
It seems to happen at random times, sometimes when loading the first file and sometimes 20 minutes to an hour into playing a playlist. Sometimes it will correct itself sometimes it won't and its driving me mad! It also seems more likely to happen after I hibernate and if I hibernate again after it happens it will take a lot longer to happen the next time I load Windows. It also seems more likely to happen with pure audio files than with video but it does happen and it has happened a couple of times whilst playing games. Oh and the sound only comes with another signal if nothing plays there is no white noise hiss.
My laptop is a Toshiba P20-S203 which Ive had about 20 months. It is running XP Home SP2 and has a Realtek AC'97 sound card. I've tried reinstalling the sound drivers. I went on the Toshiba forum and 2 people had the same problem (but with brand new laptops) and they were told to format their HDD's which they did. The problem still occured. I would say that it was a model fault except I've had mine much longer with no problems and it's only recently that its happened. (Also I tried all the solutions in the Windows Troubleshooter with no success either.)

Any help would be greatly appreciated!!