Q: Mac Recovery questions.

I have a Imac i3 27" running OSX  that I bought in 2010.    I only use this mac really for Emails, Music, Movies,pictures and internet.  I rarely use it other than that.  but lately it has been really slow.  We went on vacation and when we got back it wouldn't turn on and I had to use the Time Machine to bring it back. 

Now I used to be a windows guy and when i encountered this problem I would just do a system damage recovery and boot it like it was straight out of the box and update and load my files and boom like bran new.  I got tired of this and switched to mac.  Honestly 4 1/2 years is awesome with no issues.

Is there something that I could to bring it to the speed like when it was purchased.  My family uses it more than I do and am concerned they might have downloaded a virus.  If that is the case is it possible the virus is in my time machine back up as well.  I did a little research and apparently you can do a recovery on the mac without affecting the files you have on the computer.  should I do this or just boot like new?  And if you think booting like new is the way to go how to I do this.  I would also like to add my pictures and music manually back to the computer rather than using time machine just in case I do have a corrupted file. 

Please keep in mind I am a novice at mac but I am computer savvy

Sorry this is a lot to take in and I jump around a lot but any feedback will be greatly appreciated.