It's fair enough being a little concerned about major changes to your computer. For sure, on the odd ocassion, things can go wrong for a (usually)small percentage of users.
SP3 is a fairly big update to apply, and there is a chance you'll need to get the latest version of the Windows installer to apply it, but that's something to be aware of for the future, if the file doesn't run.
Step-by-step suggestions can be posted, if you like.
Do you happen to have Ccleaner installed?
Did you run the defragger linked by essexboy?
Did you download the tool (Startup lite) linked by Pondus?
(The above are not instructions/suggestions; I just want to know where you're at, so far. And please don't bother about apologizing for apparent "ignorance", everyone (yours truly included) is ignorant about all sorts of stuff. Being ignorant of something and
not asking...now that would be a bit stoopid.