Your English is fine.
I don't know what might be in your Windows\Install folder it could vary from system to system. On my system I have Windows\Installer and I have mainly activeX Control Folders and a lot of .msi (MicroSoftInstallation file Type Windows Installer Package).
It is not unknown for malware to place files in unlikely and or system folders to confuse users into thinking they are important files.
Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate.
What is the malware name, the infected file name, where was it found e.g. (malware name, C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections.
There is by all accounts a bug in Zone Alarm that when run in Vista it slows internet and local network access. This isn't just effecting users with avast but is possibly more noticeable because the Web Shield monitors HTTP traffic. Currently there is no ZA solution and nothing the avast can do to get round the problem as far as I'm aware. Some have suggested trying another firewall, though there aren't that many Vista compatible filewalls available, especially when it comes to free ones.