What are some tweaks to Windows 7 that can be done for a HTPC?

Since your machine is just for video playback, here are a few tweaks I would suggest to keep things speedy:

• Disable the Aero Themes.
• Disable UAC
• Disable Windows Search Indexing.

These are mostly just background processes we don't generally notice when running, but It will give you a performance boost.

You can find a few more things HERE.

Another option is tweaking the video playback itself. If you use CCCP - Combined Community Codec Pack, you will have greater control over the playback settings.

You may as want to try the DivX decoder as well, as it uses Hardware accelerated decoding.

Do note, I am not sure how well CCCP will work with XMBC or Media Player, it seems to only apply to certain video formats/players.

I also suggest using the latest GPU drivers for whatever card you are using. I know both nVidia and ATI's Control Panels have settings to control the quality output from the card.

Audio playback differs for each person mostly based on their speaker setup. A lot of computers use Realtek audio now, and they have drivers that provide different options for you to optimize it based on the room, etc.

For internet streaming, my only suggestion would be to optimize your connection. Close all programs that use an active internet connection, and make sure no one else in the home is using it. My knowledge on that topic is limited.

Media center has VERY limited audio setup, so external audio drivers are necessary for better sound playback.

If you use a graphics card or a motherboard/cpu with integrated graphics that can be used to decode video codecs and pass the files to it via the DXVA decoding, there will be few tweaks required as your processor will pass all the hard work off to the GPU.