Welcome to LQ!
1) Don't worry about spyware - it isn't a problem on Linux. This is for various reasons that you can search the web for.
2) Again, don't worry about defrag. Linux filesystems do not suffer from fragmentation like FAT / NTFS does. This is because of the way files are allocated to the disk, etc. See
Here for a good article on the subject.
3) To install programs, either use Yum on the command line, or use "Add / Remove Programs" in the menu. This will let you choose from literally thousands of programs, and will automatically download and install the program and any dependencies for you.
4) To change themes: I take it you are using Gnome (default on Fedora). Simply click System - Preferences - Themes, and select one to install. You can install new themes from
www.gnome-look.org.
I hope this helps
--ian