well, lots of nice theme's. I duno if any work/require Plus. I'm posting this in Linux, and don't the "link" button now in posting messege's now :/ . But anyhow, copy paste this link -
http://www.lightstar1.com/index.htm , on this page, you will see some of the best Theme's anywhere. On this site, there are instuctions on how to use / apply these theme's (Please have a look at FAQ before playing with the themes). Note: different instuctions for XP and XP Service pack 1.
Once set up properly, you will be able to switch from theme to theme rather easy.
Now this is more like a dll patch, not a application. The applications you Buy do this very same thing, this is just doing it manually so that you can use theme's you get off the net.
Print out instuctions and make of the files it's telling you to change if your nervous about it. But it's really quite easy.
Another good site is "themeXP.org or themeXP.com", but I think you'll find most of all the good ones there come from LightStar.
good luck, make ur desktop look the way you like, for free
