Ok so ben is right, and there is a way to exclude the file with avast. You have to open the main avast window, click antivirus tab and click on file system shield. Then click settings, then exclusions. Add the file path to your skyrim directory, and it should work. Bare in mind the skse_loader.exe will only be allowed in that folder, so when you extract it you'll have to put it in there. I'm hoping most people already have the .rar for skse from previous download, then its easy. As far as im aware, you cannot exclude individual files with avast. Thats all ive got at the moment, but it is a fix. The shield log should show that the skse_loader.exe was blocked.