Hi,
The file linked above will allow the unpacking of PowerPacker files, but depending on the content, you'll probably need additional software to make use of what's unpacked.
If you're looking at graphics files, Paint Shop Pro and IrfanView will both be able to convert from the Amiga's IFF format to something more PC-friendly.
Plain text files should be readable "as is", though the Amiga and Windows use different ways of indicating the end of each line. However, something like Wordpad will open these regardless.
For music files (modules), DeliPlayer should cope with whatever Amiga stuff you throw at it.
Word processing files and anything else created using proprietary packages on the Amiga side of things will take more work to be usable, though...
Cheers,
- Ali