My Laptop Is Being Attacked My A Viscous Trojan… Halp?

Long story short, I picked up a Trojan somewhere along the line. I knew it, because my computer was acting odd. I realized I had forgotten to download Microsoft Security Essentials, so I immediately downloaded it, and ran the scan, found the trojan, and destroyed it. It said I needed to restart my computer to complete the clean up, so I did. Upon restarting, my computer informed me I could not reboot because I may have destroyed some necessary files. I did a rollback, redid the process, but instead of destroying it, I tried to quarantine the Trojan. I had the same issue.
So I rolled back, restarted the process AGAIN, and this time when I scanned the trojan was not detected. I thought I successfully [somehow] got rid of it, though it didn’t make much sense. Just recently, when booting up, my computer said that something was wrong, so I had to MANUALLY roll back the system. Again. This time, I went to download Security Essentials with the plans of researching the precise trojan and how to get rid of it. The only problem being, I can’t install anything I download, be it iTunes or Microsoft Security Essentials. It downloads fine, but then I am informed something is wrong with Window’s Installer, and I should reboot [which I did and did nothing.]
So… What should my next move be? Haha…. Thanks for your help.