Hi Springrobin,
Look at the options and uncheck the things you don't want it to do - don't think any of them will be disastrous, but it can be annoying to cleanup some things.
In the Cleaning section, look under the Applications tab and uncheck the Windows Defender box - otherwise it wil clear the scan history and Defender will keep telling you it hasn't scanned.
Also , you may not want to clear your Internet Explorer History every time, e.g. if you want to use the History tab in IE to revisit a site later that day. It will get enormous if you never clear it though.
The Registry is a database in which Vista and your programs store their file locations,configuration settings, and so on.
It is debatable whether cleaning this confers any real advantage, and deleting the wrong keys can cause all sorts of problems, which is why you will always see advice to backup your registry before you do anything to it.
If you don't feel comfortable with that, don't use the registry cleaner and uncheck the box that says Registry Integrity, just to be sure.
Having said that, I use the registry cleaner regularly and have never had any problems.
Hope that's of some help
SIW2