Hi every1! What happens when i clean my System Registry? Does anything serious Happen? Will t affect anything in my comp?? Please help!

I need a reliable and safe registry cleaner. I downloaded some bogus software unware they were trojans.

i have downloaded INTERNET DOWNLOAD MANAGER the free trial version then after i have used it for a couple of times it expired then i have made a COMPLETE uninstallation
but i have noticed that the connection speed decreased significantly so how can i repair (restore) the changed registry configuration back to default ??

What is a good free program which can clean my PC or registry?

I hope someone can help me with this. I just added McAfee Total Protection software to my computer. Now when I restart my computer I receive the following error message: Object doesn’t support this action line: 19 misp://McRegUpd.dll::JClassFactory.js

What can I do to fix this? It’s just annoying to see this message pop up everything my computer starts.

Thanks.