I have seen problems with iTunes on many windows machines (not all from HP). That means iTunes is definitely not a very cohesive software for Windows platform.
I can understand your frustration, iTunes sometimes make my machine fully unusable; it captures all my resources.
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The PC had a few problems when I got it and I had to sent it to tech support but i got it back on Friday still as bad on the audio side. I'm frustrated beyond belief... Everything has been deinstalled and reinstalled inc. the OS!! Help please..
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I am not sure what help exactly you need...

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to stop me throwing this out the window and kill some innocent below..

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Better you put it on a curb; I bet, you will be able to get rid of this in few minutes...:)and you will save a life too...:))

I have the same problems with my HP touchsmart athalon, Itunes and media, slows, jumps, buzzes. After complete re install and getting rid of suspicious back ground files ( cyberlink ) was slightly better but still was affected, also note the visual effects of itunes was jumping and slow.
Then one day I switched off wireless adapter, it worked. everything back to normal.

I suspect The wireless network adapter is the problem, the signal strength is poor only 2 bars and perhaps the device is over compensating and thinking about it this is since i changed broadband supplier and router,. From Sky to Virgin.
My laptop does not have the same problem, intel i5, both are running windows 7 64 version.

perhaps the adapter needs new driver or someone at HP needs to get behind this problem.