Slow To Start - Destructive Process

my problem is that Windows XP is slow to start. It has gradually been getting slower since I started using it in January 2008, but last week it took a significant turn for the worse (about five minutes to load). Once it is up and running, it seems okay, although on occasions recently the mouse cursor has lagged behind its actual position. I am worried that malware may be the cause of the problem; particularly because of the sudden downwards jump in performance.

I have run the Windows Cleanup facility and the Defragmenter a couple of times. I have deleted files in various temp folders. I have McAfee VirusScan Enterprise 8.5.0i running, and I ran a full system scan, which returned nothing. I also ran scans with Spybot, Lavasofts Ad-Watch, Spyware Terminator, McAfee Stinger, and Trend Micro on-line scan. All returned nothing. I ran CCleaner, and it cleared out some junk. I have reviewed HijackThis output files (in ignorance). I manually turned off a few startup processes, and that seemed to improve matters a little. I still feel that I am failing to deal with the real issue though. I am edgy about turning off more processes, because I'm uncertain of what they do. I can refer to bleeping computer.com, and similar sites, but it is not always clear whether a process is honest, or it is a destructive process with an honest name.