For 10 years old with limited RAM, it has done well.
More RAM would greatly improve performance. Without that, constant management of the system like you already do is going to be the way.
Norton may not be the best security product for this machine. As Norton upgrades over the years it requires more resources, and on your machine they are not available. There are free security applications available, and the ones that 'only do one job' might be best, eg a separate firewall, a separate anti-virus, and a couple of small anti-spyware applications. They then only request resources when they are needed.
You could check your Virtual Memory settings. On your computer I would let the system manage the size of the Page File, but low hard disk space may affect how much virtual memory is available for the system. If that is the case you might be able to free up disk space by disabling System Restore, then re-enabling it.
Other than that I think you're doing well with what is, in effect, an older system.
Okay, I have a 10yo Dell with 256 MB ram. My CPU always climbs to 100% despite turning off all non-MS processes on startup. I open task manager and nothing in the CPU column averages more than 4-6 except Firefox (40-50%) when running--- often using 100,000k with only two tabs open. I run Norton daily with a comprehensive scan, clean my registry, optimize my drives daily, and clean out the temp folders hourly. Could it be that my CPU usage is at 363 MB and relying on virtual memory? I'm a neophyte here so don't laugh at any unintentional ignorance.
What can be slowing down the machine? I cannot find what hidden processes might be running. I have the token 5 svchost files, 2 ccSvchst files, and 15 other processes running. Idle processes is often at 92%, which is confusing if the overall usage is so high. I turned off all automatic MS updates and rolled back a few. I set all my options to adjust for best performance. Blew dust off the fan area.
Programs are slower this week. Firefox tended to crash 3-4 times per session. That seems to have cleared up when I uninstalled/reinstalled Norton, which now takes 3 hrs+ for a comprehensive scan.
Any ideas or suggestions? Please don't say my system is starting to go as I'm in the 10% unemployed right now until hiring freezes are lifted by school systems.