There is not a lot of info in your questions but here goes.
IIS can record log files in various types of formats. You can specify the format using the IIS MMC. Right click on the site, properties.
Log files are just plain text which you can view using any editor (notepad for instance). Log file analysers just parse the file and extract meaningful information from them.
One "trick" with log file analysis is to make sure that the program can understand the format of the log file. Not all of them can read the IIS Log File Format.
Check which format you are using and then check to make sure that the shareware app or demo is actually capable of reading that format. It may be trying to read an IIS log file as say a W3C file format.
Regards
Raffi_