Plenty of things you can do, although the best, quickest way is to format your computer and just reinstall Windows.
If you don't want to do that -
Start > Search: msconfig - Services tab, tick the 'hide all Microsoft services', Then just go through and untick whatever you don't want to start up when you start your PC up. So for example if you had Malwarebytes set to start up automatically and you untick it, it won't start on startup so you'd have to do in manually.
Then do the same on the next tab 'startup' and untick whatever you don't starting up with your PC.
After that close it (ignore the restart message).
Start > Search 'disk cleanup'. Let that run and then you can tick whatever you want to be deleted from your PC. If you go in the next tab you can set it to delete all but the most recent system restore point, although only do this if you're sure you won't need to restore it anytime soon.
Start > Search: Add or Remove Programs. Go through the list and uninstall whatever programs you don't want on there. The more you can remove the better.
Start > Search: Windows Update. Check for updates and install the important ones and some of the optional ones, these don't necessarily speed your PC up but they reduce errors and crashing that occur on it.
You can consider this -
http://www.majorgeeks.com/Advanced_SystemCare_Free_6.2.0.254_d5927.html
Description of program (but use the above link to download it, it has an updated version -
http://download.cnet.com/Advanced-SystemCare/3000-2086_4-10407614.html
This program is basically an enhanced version of disk clean up, except it cleans out and gets rid of a hell of a lot more junk that the Windows one does. If you choose to install it then be sure not to install any of the freeware crap they try to sneak in with it. There's a free version and a pro version, if you want the pro version then I can give you a staff to use (which I am allowed to possess and give out) you'll have to PM me for it though. Be sure to go through the options on it so you know what will be deleted and such.
Finally -
http://download.cnet.com/Smart-Defrag-2/3000-2094_4-10759533.html
This is an enhanced version of Windows Defrag, it does what it does a lot better than the Windows one. You can choose it to move your most frequently used files to the fastest areas of your hard drive. So for example if you play COD4 a lot (just an example) then it would move COD4 to the fastest area of your HDD so it loads faster and can be accessed faster on your PC.
Try thouse ^_^