Just upgrade your hardware, cleanup junk stuff & uninstall programs that you're not using, stop those useless startup programs. That's typically it.
Absolutely, you can't just buy a new PC.. Lol, that is just lame (BUT of course if you want something new and you have the money then it's your decision!) I believe, you can try this first before purchasing a new PC! It might also be that your computer is overly used!
check this: poor Random Access Memory overload with documents, pictures and videos has computer virus.
1 Buy faster computer (if you can afford it) 2 If money is tight buy more Ram 3 Dont defrag, but format HD and re-install software.