Spybot has fallen out of favor as it's teatimer can conflict with many security suites. I suggest you uninstall it and move on to one of Grif's favorites listed at:
Your aversion to opening the case is a clue the machine is not getting it's yearly cleaning. Here's a video on that:
You write about many common software scans but XP on such a machine can degrade the IDE mode. Don't fret as this is easy to solve and no reboot required as noted at link:
Delayed folder and file manipulations using Windows Explorer (not Internet Explorer). If I try to download anything, it?s seems like the system is looking a long time to open the file system. However, once a file is downloaded, it downloads normally. Slow finding and saving documents from MS Word. All programs from desktop icons slow to open. Once in a while, the letter display doesn?t keep up with my typing. Sometimes II get a pop-up saying the script has stopped responding, then the computer hangs.
Make: Hewlett Packard
Model: Pavilion 750n
Processor & speed: Pentium 4, 1.6 GHz
SD/RAM: 768 (upgraded form 512 MB)
OS: Windows XP home ed with SP3
Used space ? 17.3 GB, free space ? 57.1 GB
Do not know anything about internal hardware ? uncomfortable about removing cover and looking inside if I even knew what I was looking at. I bought this used desktop system a little over a year ago, but the last few months have had problems with slow operation. No disks were included, so I can?t re-install OS or run scannow.
Computer Associates security suite includes anti-virus, anti-spyware and a firewall
SpyBot updated weekly and scanned monthly
CCleaner ?scanned daily
ATF cleaner- scanned several times a week
I do not have AdAware ? I tried it and it too so long to download and run, I uninstalled it.
I use mainly Firefox (currently 3.6.12), Internet Explorer 6.0 also available, but not used much
What I have done so far
Run all scans with security software.
Defrag of hard drive
Ran Windows error checking program
no viruses, spyware or other errors discovered