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.