What's wrong with my computer?

I built a computer about 6 months ago, installed windows 7, and recently it's been having problems. ever since I installed windows the sound will not work after waking up from sleep, but only sometimes, and it works if I do a restart. the internet is sometimes a lot slower than other computers on the same network. And this next one has only happened twice, the hotkeys in starcraft 2 don't work or do different keys functions, but when I quit starcraft they work again. I have a quad core AMD proccessor, 4GB of ram, and a Nvidia Gforce GT240 video card, but a hard drive from a really old PC. what could I try to fix this? would reinstalling windows or buying a New HD help?