There are several factors:
1. it can just be that everything mechanical/technical will slow down with age.
2. The hard drive gets clogged with rubbish, defragging can speed things up a bit but so can removing old programs that arebn't eneded or wiping and doing a clean re-install.
3. Modern programs have higher demands, therefore and older system will run them slower. If you had version 2 of a program and then got version 3 you may find that it runs slower because it has higher requirements.
4. Adding additional memory to an old machine is the cheapest way of speeding it up without replacing the processor/motherboard etc which can be costly.
Corm