Okay, I had to make an account just to post a reply to this. The registry fills up with errors as you install and uninstall programs. These leftover entries and errors are (mostly) harmless, but they can accumulate over time and bog down the system. It is perfectly harmless to remove these entries, but it is tricky without the proper software. I've found that Ccleaner is a great program for cleaning out registry entries. It scans through the registry, requests for you to export a backup, and then allows you to remove the entries. That works for some issues.
After I uninstall any programs with the intention to never reinstall them, I do a registry search for entries containing the name of the program, as well as the name of the company that makes it, and any other related words. It points out any entries with those words, and I judge wether or not I should delete them. I've only messed this up once, which killed my firewall. A minute later, it was already restored and the firewall was up and running. I've never had an issue since, and I've deleted hundreds of entries.