Problems with Win7 64 bit on iMac i7 - slow & noisy

My iMac i7 from April 2010 is dead-slow in Photoshop CS4 on Win7 64 bit.
I work on a simple photoshop document (approximately 2000x2000 pixels) with guidelines. I hide the guidelines and unhide them again, and it can literally take several seconds for the computer to make them appear.

• Windows is in general slow, often "lags"
• Makes noise at odd moments (eg noisy consistently when the screen is on standby and black).
• The fan is sometimes very high, and the hard drive too hot (see attached graphics)
• The brightness of the screen can hardly be adjusted (it hardly gets dark).

In the shop they now say they no longer recommend Windows 7 64 bit, because they have had problems with it.

I am running bootcamp 3.1, updated as much as it can. Have also updated to the latest Parallels, but it also gives errors (I must constantly authorize Windows, and Parallels drivers appear as broken in Windows). I have uninstalled Parallels, but it does not help.

I have enclosed two graphs - one showing that the hard drive becomes very hot, and the second is a test of the graphics card which I think looks strange.

Can you help?

Best wishes
Chester