To obtain Service Pack 2, go to Windows Update. You will be offered
Service Pack 3, which includes Service Pack 2.
It's extremely important that you keep your computer up-to-date. If you
have not previously visited Windows Update, you will likely be offered a
long list of updates. Set aside some time - about an hour - to download
and install the updates. You may have to return to Windows Update
several times to get all the updates you need.
I suggest you go to Control Panel > Security Center > Windows Updates
and, at the very least, set yourself up to receive notification when new
updates are available. Install the updates as soon as you are notified.