Volume Problem: To see if you lost any drivers, just go to "Windows Update" and try to see if you can get it there. If that doesn't work, try finding new hardware and search for your sound drivers online
Slow Computer: It could be a virus, but it's probably just spyware... download "Spybot: Search & Destroy" to get rid of most of it...if it's still slow, maybe you need to get a new virus scanner
Everything in Enlarged: Try changing the resolution, but since it happened hte same time you lost your sound, i'm gussing you accidently deleted your video drivers as well. You should reinstall your video drivers and change your resolution
Good Luck