Excessive Startup Time

I use a Windows XP Sp3 desktop computer with an AMD Athlon 2GHZ processor,
149GB hard disc with 133.3GB free space and 512MB of RAM. I regularly use
CCleaner, Ad-Aware and Ashampoo Cleaner. After startup, when all the desktop,
task bar and notification area icons have appeared, the processor runs for
another 5 or 6 minutes before I am able to use it. In msconfig Startup, I
have turned off all the none essential programmes I recognise to stop them
starting but it makes no difference.

In Task Manager, quite often, System Idle is showing CPU use as 92% to 99%
idle but the hard disc is very busy with CPU use diving between various
processes but AAW Service.exe which I believe to be Ad-Aware disabled in
msconfig, is always shown as working and sometimes takes over from System
Idle with up to 99% CPU usage. What is going on?

Other than completely re-installing Windows, can anyone please suggest what
I can do to speed startup?