An interesting Vista bug has been brought to light that many have wondered about, but had not yet until now been confirmed. Mark Russinovich has documented the steps needed to reproduce a bug in Vista in which you can cause the machines networking capabilities to slow down or even halt altogether, due to a Vista feature called MMCSS.

This feature was intended to improve the quality of your “media experience” when playing audio or video. It may have accomplished that at the cost of other resources:

Its mission is to automatically prioritize the playback of audio and video to prevent skipping, sputtering and other unseemly glitches. Unfortunately, it ends up fixing a problem that was largely non-existent and leaving in its place a major headache for many sysadmins.
The bug is such an issue, it seems, that even playing files as innocent as the Windows error beeps can cause serious network degradation. The problem is also seen in games and meda suites such as Real Player.

Just how bad can it be? Apparently, it can be a network killer:

Russinovich documents how a throughput of about 20 percent on his 1Gb local area network dropped to about 6 percent simply by trying to pass the time by playing a tune in Windows Media Player. Effects like that will also be felt on 100MB networks.
This is definitely interesting. According to the article, Microsoft developers are at least aware of the problem. There is no official acknowledgment of the issue yet, however, which has been a bone of contention for some. A workaround has been posted that seems to cure the issue, by simply disabling MMCS. That's not, of course, an official fix.