Ok, #1 where to find OTS themes, and two! I thought the ones off this site were OTS's and when I used the Install Program to install the theme it said it needed some moduals and wanted to know if I wanted to d/l them I did, after that it gave me all kinds of errors of not being able to either initiate or end the modual properly and would I like to return to the default theme, I am running windows XP
#1: all over the web. ;) Most themes currently released are OTS compliant. Or rather, claim to be. See #2, below.
#2: unfortunately, many themers don't really care to test their themes on different configurations, just add the OTSMajorVersion / OTSMinorVersion lines, that's it. Hence, you may have to fix the theme for the bloke who did it. :( Make sure to check comments to different themes, e.g. on litestep.net, if you encounter any issues with running a theme. Also, some modules have bugs which may produce weirdo effects such as random litestep crashes and the like. It's still pretty much trial and error, really.
Just read the forums and especially the readmes that ship with themes you download, and you should be fine.