Just my two cents worth in addition to what's already mentioned...
By no means is this a complete list of things to make your computer "fresh" or "run faster", but I've found the following helps, in this particular order:
1. Check all your installed programs and un-install any not being used.
2. Run your Disk Defragmentation program (bundled with Windows O/S, if you use another O/S there should be a similar one, or you can download free ones online that are safe to use).
Over time, your harddisk data becomes fragmented, so that when it tries to run a program, it has to look in several different places to "piece" the program together before starting up. Defragging your disk mitigates this problem. I usually try running it once every quarter (the program will analyse and let you know if you need to run it).
3. If you feel comfortable, start up a command prompt and run your Registry Editor (type "regedit"). This lets you get rid of registry entries which un-installed programs may have left in the registry, even after you have un-installed them.
This slows your computer down when you start it up, as the O/S has more entries to go through...
Also, you may want to check if there are any programs which the Registry starts by default when your turn on your computer. If there are, and you don't need them, you can remove their start entries, and this saves you a bit of time.