Windows XP performance slowdown

Is it just me, or does Windows XP seem to get slower with every update? I have noticed that, even with the bare minimum of programs running (AntiSpyware and AVG), a decently fast (2 or more GHz, 512 MB) machine is just not what it used to be, and boot times seem to be longer than they once were. This isn’t confined to a single machine. Installing a fresh copy of XP does not seem to help once the required updates and security measures are loaded. These machines have decent specifications, but just don’t pass the “seat-of-the-pants” test for startup time. Am I hallucinating?

Best regards,
ncrenshaw