As a tech writer, one of my biggest pet peeves is the plethora of bad advice littered across almost every web site dedicated to system tweaking. Besides the tweaks that simply don't work, some of them will actually cause your computer to run even slower—or worse. Let's examine some of the most offensive myths out there regarding PC performance tweaking, and debunk them once and for all.

This tip made the rounds with people believing that Microsoft always allocates 20% of your bandwidth for Windows Update. According to the instructions, you were supposed to disable QoS in order to free up bandwidth. Unfortunately this tip was not only wrong, but disabling QoS will cause problems with applications that rely on it, like some streaming media or VoIP applications.

Rather than taking my word for it, you can read the official Microsoft response: "There have been claims in various published technical articles and newsgroup postings that Windows XP always reserves 20 percent of the available bandwidth for QoS. These claims a