It is for sure that every computer will become slow as days by and other kinds of causes. If it is an old computer, if possible, you can try adding an RAM modules, or update graphics card, or purchase a whole new computer package to directly speed up the computer. But you can try the normal tips below to speed it up without any cost.
1) Uninstall the programs that you don't need.
2) Delete the files you don’t need
3) Use the disk clean-up option in Windows to regularly clean up the system junk files.
4) Turn off System Restore feature.
5) Use a reliable registry cleaner to clear the windows registry.
6) Regularly scan the computer for viruses and spyware.
7) Remember to disable some unneeded startup items.
You can see more information here: http://ezinearticles.com/?Slow-Computer---How-to-Fix-a-Slow-Computer-Easily&id=3643022.