I just built my own computer and all I have on it is ESET nod 32 (antivirus) 2 Adobe trials, a game, mods for the game, and steam.... Last night my computer was VERY fast but today it seems to be running slower and youtube videos will NOT load (some just very slowly). They (and everything else) was great yesterday. And to top it off my game even froze when I quit!
Start over. If it doesn't have that much on it, then back up what you want to keep or just reinstall it all, but if it's that noticeable then something happened. And the longer you live with it, the more of a pain it will be when you can't solve the problem. Format your drive, reinstall the os and just start over. If that doesn't solve it then you have a hardware issue. If it does, then there probably was a more elegant solution, but who cares if you only have like three programs to replace? Hope that helps. Good luck.