As that screenshots says, it is "not a virus". The only way mIRC could cause a slowdown is if it's running. If it's not running, it cannot cause any kind of slowdown. Most likely, restarting your computer just helped to clean things up for you.
When was the last time you installed Windows? Windows (any version) works best if it's reinstalled at least every 1-2 years. Failure to do so will cause noticeable performance issues.
There are other viruses and trojans out there that most normal antivirus programs will not detect. That's why you should always run antivirus and regularly scan using at least one anti-spyware program. Running Hijackthis is a good additional step as it checks any possible issues regarding browser hijackers.
