Windows Performance Issues

I have been having a performance problem with one of my Windows XP computers. All of a sudden, the computer has been running slower. When diagnosing a problem like this, the first thing you want to do is determine what resource is the bottleneck, then what program is consuming too much of that resource. This particular computer has 2 cores (of an AMD X6) and 3GB (of 16GB) of RAM. When I fired up Task Manager to figure out what resource was the bottleneck, I learned that it was the CPU. The computer was using 100% of each CPU and only 300MB of RAM. After waiting a while, the CPU usage dropped down, but the idle system was using 25% of each CPU! That is just wait too much CPU utilization for an idle system. I don't run anti-virus on the system, because I am very careful about what I put into the system. The only other Windows PC's on the network are either heavily anti-virused, or I am also very careful about what I install on them. That lead me to believe that it wasn't malware that was causing a problem. I turned off all the visual items in the Performance tab. That increased the responsiveness of the computer a noticeable amount. I disabled System Restore since I use QCOW2's snapshoting feature. I looked at the process table in Task Manager and started Googling what those processes were. Some of them were not needed, so I started uninstalling software. I looked up Windows XP tweaks and found a few good ideas (other than the normal "upgrade your PC"). I disabled the index service. I disabled some services at startup. I disabled "Last Access Time" on the filesystem. I disabled the disk performance counter. I deleted the Preload, but I don't think that will help me. Deletion of the Preload folder on a monthly basis is based on a sound theory, but it assumes your PC is a general use computer. My PC is for gaming, so there was very little in the Preload folder to begin with. I did a disk cleanup, but that didn't have a lot to clean up. It did offer to compress a bunch of files to save 2GB, so I decided it was worth it. That will help with the snapshotting process. I finally broke down and installed LavaSoft's Ad-Aware. Normally I hate having anti-virus programs installed for two reasons. First, they often advertise heavily and add pseudo-malware like browser toolbars. Second, live scanning tends to consume a lot of resources. Live scanning tends to make your system slower to the point where it feels like you have a virus installed. I ran a scan and it identified one quarantined item. I thought to myself, this is it. I must have installed something that added malware. I clicked on the link to view the item....and it was tracking cookies. Eleven tracking cookies to be precise. Looks like you can protect a system from viruses with your brain as opposed to pseudo-malware. I ended doing what some people recommended and "upgraded the hardware". I vi'ed my xen configuration file and added a CPU. After all that, Windows is still running slower than it used to. This is why I use Linux.