Adding themes to windows vista is very simple and can be done on all versions (Basic, Home Premium, Ultimate).
This can be completely reversable.
This will not harm your computer.
It may slow down your computer depending on your memory.
Step 1: Tuneup Utilities
First thing your going to want to do is download Tuneup Utiilities.
This Program is very useful and is the key to adding themes to your computer.
This program is useful as a registry mechanic too.
Vista Glazz is not needed. Most guides require it.
Step 2: Finding a Theme
If you use Windows Vista, MAKE SURE the theme is for vista and not Windows 7.
Same for Windows 7.
You can't mix these.
When you download your theme, make a new folder named themes to keep track of theme.
Step 3: Final Step :)
All you have to do to revamp your Vista/7 System with an awesome theme is open Tuneup Utlities/
Now click Customize Windows/
Then click Visual Style
After that, click the +Add Button and choose your "Themes" file on your desktop.
Choose the theme you downloaded and apply.
Thats how its done. Quick, free, and very easy.
If you had any trouble, restart this guide and try again.
You may need administrative rights.