Not just that but why do an uninstall and re-install of avast. ??? I would think a “repair” would suffice. 
At least the OP has the issue behind them. 
@emilemma try using a cleaner like CCleaner http://filehippo.com/download_ccleaner/
You will find CCleaner is very user friendly. 
avast will not scan the registry only files which can be infected. To scan the registry use CCleaner.
Rootkits; avast by default does a quick scan. You do have the option to create a "custom scan. On a custom scan avast allows the user to set certain parameters. I would suggest leaving the packer setting at the default setting. If you do a full scan for rootkits you may get some false positives. For the most part the default full system scan will be sufficient. I do a full system scan once a week and a quick scan twice a week. To do more than one full scan a week is overkill due to the fact that avast has some very strong resident shields. That is unless you play with fire, then you might get burned. 