After installing Windows 7 Ultimate 64-bit on a stock HP Pavilion Slimline s3750t my computer would completely freeze up or have other issues.  Hopefully this will save someone the aggravation I went through.

Symptoms:

1.  20 to 30 minutes after boot all USB devices stop working.  Wireless adapters and other USB powered devices will loose power regardless of the USB port they are plugged into (front or back).  Unplugging and reconnecting devices has no effect.  Once this occurs if you go to Device Manager -> Universal Serial Bus Controllers -> USB Root Hub (the one attached to the PCI controller, on mine it's the last one in the list) and select the Advanced tab Microsoft Management Console will stop responding.  The problem will clear after a power reset (until it occurs again).

2.  Windows will completely stop functioning (freeze) either minutes or days after startup (the clock in the taskbar will stop updating).  A manual kernel panic (right cntrl-scroll lock - scroll lock) is not successful.

These symptoms occur if you do an upgrade from Vista 64-bit that came with the machine, a clean install using only the drivers which come with Windows 7, a clean install plus the latest (as of Nov 3 2009) drivers from the HP support website, or a clean install plus the drivers from nVidia themselves.

Workaround:

Disable the USB Root Hub associated with the PCI to USB Host Controller.  In device manager you will see two.  Disable the one that lists its location as "on Standard Enhanced PCI to USB Host Controller" in its properties screen.  Once you do this you'll only be able to use the USB ports on the back of the machine (media/flash card reader also goes down) but at least your machine will stay up.

HP, any word on when a fix will be out for this? 

You may want to try installing the newest Chipset drivers for Windows 7 directly from nVidia.

http://www.nvidia.com/object/nforce_vista_win7_64bit_15.49.html

I was thinking the same thing.    I tried a clean install with the drivers I picked from nVidia, then with the ones their "wizard" suggested.  Still no luck. 

Did you try installing all the optional drivers available on Windows Update?

windowsupdate.microsoft.com