The jv16 PowerTools 2005's registry cleaner lists only items that it thinks should be safe to remove. So, if you have trust in it, just select all items, click the Fix button and hope everything goes well. It almost always does.
However, it's recommended to quickly browse through the list of found entires. You don't have to spend hours with the list, but usually takes less than a minute to quickly go through the list of found items.
If you are unsure if some items are safe to remove or not, you shouldn't remove them. That's usually always the safest thing to do.
About the error severity rating from the handbook:
"The error severity means how probable the error is to cause problems in your system. The error
severities should be interpreted as follows.
High - The error is quite likely to cause some system or user level problems. Typical such system level problems are error messages upon system startup or system crashes. User level problems include applications to stop working or start to show error messages of missing files or components when used.
Medium - The error could cause some problems but that is not very likely.
Low - In theory, the error could cause some minor user level problems but it's quite unlikely.
Lowest - The error is very unlikely to cause any problems whatsoever."
