Add more RAM
That is the biggest speed increase you can do in-situ. The hard disk is the slowest device in your PC. More RAM will help prevent it from thrashing around.
Install an SSD drive
These are much cheaper and more reliable now. Ideally you would want to reinstall the O/S on it but you can still see a big boost by adding it as a second disk and reinstalling your apps onto it and moving your data onto it. Leave the existing disk purely for the O/S
Uninstall Crap
Most ready made PCs come preinstalled with bloatware. Uninstall them. Also look out for crap that came with free software that you may have installed. If you only use a program once a year then uninstall it and install it once a year!
Get rid of anything by Adobe, Java, and free internet security suites such as McAfee, Norton, Symantec, which are supplied / bundled with other apps.
Adobe is a big hog. Certainly get rid of the PDF viewer and use one of the more lightweight free apps. The only Adobe app you need for browsing is Flash Player.
Java is the biggest risk to security in your PC. If you don't use any apps that require Java then ditch it. If you visit a website that requires Java then seriously, find another website.
Uninstall toolbars, which are added as freebies. Uninstall all the fluffy bunny guff that comes with printers.
Take a look at the start tray icons next to the clock. Ideally you only want a firewall and AV icon there. Anything else will slow you down at startup and whilst running.
Change Browser
Dont' use Internet Explorer. Use Firefox, with add-ons such as FasterFox for optimisation, and other add-ons such as Flashblock, ghostery, adblockplus and other anti spyware / adware add-ons.
Try Google Chrome or Opera. You can install them all at the same time if you wish.
Services
Take a look at what services are running at startup. Search for guides on which services you can safely disable. Note that you need the correct guide for your O/S.
Turn Off Restore Points
This is stupid. Install a proper full backup program like Macrium Reflect. Windows System restore is more hassle than it is worth.
Turn Off Screen Effects
Try changing settings to Performance rather than Quality. This means turning off things like shadows under mouse pointer, viewable windows whilst dragging etc.
Clean PC
Dust is a big problem. It can block heatsinks and cause fans to be less efficient. If your CPU is getting too hot then it will slow down in order to reduce heat. Regularly clean all dust from inlet and outlet fans as well as the CPU fan. Caution advised when doing that.