You really should use Windows Update. Windows Update is a Microsoft site that will analyze your system to determine what critical security and other optional updates it needs. I would install ALL critical updates, however, there will be a lot of them and downloading them will take time. Also, if you have not visited Windows Update before, you will have to allow it to install updated software to use the Windows Update site.
Service Pack 1 should be one of the critical updates offered. After installing Service Pack 1 and you return to Windows Update, your PC will be re-analyzed and new updates will be offered such as Service Pack 2. Repeat this process until there are none left. When installing various updates, I recommend doing a few at a time such as 10 to 15 or so. Sometimes when I've tried to do them all at once, I've run into problems with them installing properly so if I do them a handful at a time it's more successful.