PE Explorer is a powerful tool that provides everything you might need to inspect the inner workings of 32-bit Windows executable files.
You can view all kinds of data about your file, for instance: its header; export, import and delay import functions; any file dependencies; debug information, the relocation table, and much more.
A wide range of editors allow you to tweak the executable in various interesting ways. So you can use the Resource Editor to edit or replace icons, bitmaps, string tables, menus and so on. The Application Manifest Wizard allows you to insert an application manifest into the resource section of your file, so for instance you might tell Windows Vista or 7 to run it elevated. And there are other tools to change the PE file header Characteristics field, tweak section headers, modify the timestamps in the PE file header, and more.
A powerful, yet easy-to-use disassembler allows you to quickly explore what your program is doing at a very low level.
And the ability to work with all 32-bit PE files, combined with UPX, UPACK and NSPACK unpackers mean PE Explorer should be able to handle just about anything you can throw at it.
Please note, this trial version of the program has a few restrictions: it can't save all resources to disk at once, for instance, and it's unable to remove relocation or debug information from a file. There's nothing that will prevent you properly evaluating PE Explorer, though, and of course if you purchase a licence then these functions will be enabled.
An excellent tool, packed with features, PE Explorer is also generally easy to use (compared to other disassemblers, anyway)